This answer elaborates a bit on the answer above, pulling from some YouTube videos done by Specialized Bicycles in their own wind tunnel as well as a few other sources. It goes on a (hopefully acceptable) tangent by discussing an alternative hand position for road cyclists, some of whom may not be able (due to racing regulations) or willing (due to vanity) to install aero bars. It also discusses the potential use of aero bars in group rides - there are some situations when this is acceptable!
The case for aerodynamics
Aerodynamic drag produces most of the resistance you need to overcome when in motion, unless you are riding up a very steep hill. Anecdotally, I believe I've heard aerodynamic experts (Josh Poertner of Zipp, the folks at Flo Cycling) discuss on podcasts that the average cyclist needs to be on about a 6% climb before the effect of lighter weight overcomes that of better aerodynamics.
Quoting @RChung (and simplifying a bit), the power needed to overcome aerodynamic drag is:
0.5 * drag coefficient * frontal area * air density * speed ^ 3
In many contexts, you'll hear the term CdA, or coefficient of drag area - this is basically the drag coefficient * drag area. Aero bars and getting in the drops reduce your CdA. (Note: corrected some text thanks to @RChung's comment)
The aero hoods position
The original post didn't mention the aero hoods position. You have your hands on your hoods, but you crouch so that your forearms are parallel to the ground. That looks like the picture below, courtesy of Cyclingweekly:
This position shields your forearms, similar to being in aero bars. This reduces your frontal area, which is one of the components in the equation above. Surprisingly, aero hoods is actually more aero than riding in the drops, as shown subsequently. Consider that in the drops, your forearms are actually exposed to the wind, unless you are crouching very, very low in the drops. I suspect most of us can't produce high power if crouched that low. I've tried, and it's not that comfortable to hold. Also, when in aero hoods, I believe you are about as low as you are in the drops. Thus, your frontal area should be reduced by more when you're in aero hoods.
Otherwise, the usual positions for road cyclists without aero bars include, from most to least drag, hands on the tops, hands on the brake hoods, and hands in the drops. Some road racers may also use what I call a faux aerobars position, where they put their wrists close to the stem as if they were holding short aero bars. I recall some people doing this in the 2000s or so, and I've done it myself on occasion, but my impression is that it has gone out of style in favor of aero hoods. As your control over the bike is reduced considerably, I can't recommend that anyone do this.
Comparative savings between positions
Much of this section draws on work by Specialized. For each position or upgrade, Specialized prefer to present the number of seconds saved per 40 kilometers in a constant solo effort. They argue, in this article by Bikerumor, that a change in position or in equipment tends to produce about the same amount of total time saved between 20 and 50 km/hr (about 12.4 to 31.1 mph). That is, no matter what your own speed, you should save about equal time. The amount of power saved, naturally, varies significantly. (Image from Specialized presented in the BikeRumour article)
In this YouTube video, Specialized demonstrated that for the test rider:
- Going from bar tops to hoods was worth 90 seconds over 40km
- Going from brake hoods to the drops was worth 65 seconds over 40km
- Going from the drops to a faux aerobar position was worth 3 minutes over 40km (reminder: I can't recommend you do this!)
A different video shows that for a different rider on a different bike:
- Going from drops (without the aero bars mounted) to clip-on aero bars was worth 107 seconds over 40km
Notably, in the last clip, when the rider was in her aero bar position, she sitting noticeably higher up than when she was on the drops. (See 0:58 for a frontal view in the aero bars, 0:50 for frontal view in drops.) This was without changing the rest of her bike position. I believe that one of the commenters on the original question may have found it counterintuitive that a relatively high aero bar position could still reduce drag. It can.
This article by road.cc discusses an academic article comparing the regular hoods position, the drops, aero hoods, and short reach aero bars. This is the same rider on the same bike. I'm unable to access the article, but drawing from the text, this is the power required by the studied rider to maintain 28 mph (45 km/h) in a wind tunnel, at 0 degrees yaw:
- Hoods and upright: 430W
- Drops, traditional posture: 417W
- Drops, crouched: 385W
- Aero hoods position: 372W
- Draft legal aero bars: max of 365W, can achieve as low as 358W with crouched position
The test discussed above shows a bit over 5% reduction in required power at their testing speed when going from standard drop position to clip on aero bars, or a bit over 4% reduction from drops to aero hoods. The early sections of this GCN video corroborate that the aero hoods is a significant power savings over the drops (33.7W at 30km/h; they discussed that the rider's CdA on his road bike in aero hoods was comparable to some riders on their time trial bikes).
One issue to consider is that if you have aero bars installed but you are not using them, this seems like it should cause some additional drag, since you are adding frontal area (recall the formula cited above) to one of the leading edges of your bicycle. If you are in a triathlon or time trial and you were going to be in your aero bars almost all the time, this is obviously moot. It could possibly be something to consider if you are not in a triathlon or time trial, as discussed later. However, I'm not aware of testing on the additional drag imposed by unused aero bars.
In any case, versus the standard upright position on the hoods and the drops, aero bars are a significant reduction in drag. Roadies who can't or don't want to use aero bars may want to be aware of the aero hoods position, because it comes very close to clip on aero bars in drag. This does strain your triceps a bit, so it isn't typically used over very long distances. Naturally, one could train for this position specifically if one were interested.
Aero bars give you an alternate riding position to use
Never mind just drag: in very long rides, aero bars do offer an additional riding position that you can cycle through. This can reduce strain on your neck, shoulders, and arms over time, as articulated in this Cycling About article on bikepacking. However, many long rides involve cycling in a group at least some of the time. This leads to the next section.
Using your aero bars in groups
The following is irrelevant to the OP's, but it's worth discussing how or if aero bars should be used in groups. You lack easy access to your brakes and your steering control is reduced when on aerobars. This was alluded to in the other answer. In a paceline, you may have to brake or maneuver on short notice. Thus, aero bars are outright banned in road races. (The Cinelli Spinacis were small clip on aero bars that were UCI legal for a short time, but were banned in 1997.)
However, many longer rides, like gravel races, audaxes, or road gran fondos or century rides don't explicitly ban aero bars. Anyone considering using aero bars in these events should exercise caution and use some common sense. In particular, I would recommend staying out of your aero bars if you are in a paceline behind other riders. I would actively object to riders who use their aero bars in a paceline, although I would probably accept it if they were taking a turn at the front and they demonstrated sufficient handling skill. I would take additional caution on gravel, and I would take additional caution if there were a lot of other riders around (e.g. my group catches another group in front, or just after starting the race when most riders are together).
Geoff Kabush, a professional mountain bike and gravel racer, vehemently opposes aero bars in gravel rides due to safety issues, and he thinks they should be banned outright. The lead groups often are riding fast and in close proximity, and obviously they are mostly on loose terrain. A counterpoint by Mat Stephens, another competitive gravel rider, is available here. He essentially says that people need to get over it, and that they can save the rider a considerable amount of power expended in a long race with many windy stretches. Recall that if you save power, you also have to eat less. Of course, Stephens has presumably has good bike handling skills. And because he has outright won a major gravel race, he could probably drop most people objecting.
See Chris H's comment below on aero bars being widely accepted in audax rides. These tend to be very long, multiple day rides. I would expect the groups to be very stretched out. This particular road Gran Fondo (Italian for "big ride", generally means a big organized single day ride that isn't a sanctioned race) strongly discourages aero bars but does not ban them. This likely does not represent all Gran Fondos, but is an example of a possible step a ride could take.
In group rides organized by clubs, my experience is that I have rarely seen aero bars as road cyclists tend to prefer not to use them. I think this is a combination of fashion, plus aero bars being banned in sanctioned road races. I suspect that many clubs will lack specific rules because they encounter the situation rarely. However, a ride leader might exercise their discretion to ask you to leave. Triathlete group rides are likely to be different because aero bars are de rigueur in triathlons, but I suspect triathletes also ride further away from each other than road-oriented groups. See Vladimir F's comment about his own cycling club.
In a group situation, remember that your actions affect the people riding close to you. Because of group cycling dynamics, this can include people who did not make a conscious choice to ride with you!