The answers to this question strongly advocate for smooth tires or maybe tires with just a very shallow profile. What are mountain bike tires with deep profile good for, do they have their own important strong sides? Why are new bicycles sold with them at all if smooth tires are better nearly everywhere?
The question you linked to was specifically asked for roads. Smooth tires are indeed better on hard and smooth surfaces like roads. The shallow profile helps evacuate the water that can be between the tire and road.
By contrast, MTBs are designed to be used on loose surfaces (earth, mud, sand, stones, ...). The knobs are better in those environments for several reasons:
- on "surfaces" like stones/roots, you can only have limited contact points (the edges of the rocks/roots): the knobs can approach edges "perpendicularly" instead of "tangentially", and not rely on friction in the same way.
- they allow gripping on hard features that are under the loose surface (for example, a root under mud, a stone under sand) — that's also why they need a certain distance between them: if they are too close together, it's like if it were a continuous surface).
- on wet surfaces, friction coefficients are much lower, and smooth tires just slip...
Renaud's excellent answer can be elaborated upon regarding soft surfaces.
Since it is obvious that bicycle tire rubber is not hard enough to penetrate asphalt and the rider's weight is not a large enough force to do so anyways, tire tread will not appreciably dig into a paved road
. That's why a micro-tread pattern like seen in the following image is all you need. This tread will help engage with asphalt's existing surface roughness:
However, when riding off-road, the soil is often soft enough that tire rubber will dig in. This creates a paddlewheel effect, which gives extra traction. For reference, here is a tire made for muddy conditions with suitably exaggerated tread:
You can see how the pronounced tread will dig into the mud. As Renaud mentions, you transition from purely "tangential" friction to now having the perpendicular shear area of the soil/mud trapped between the knobs giving your input forces something to push against.
On slightly harder surfaces (eg. hardpacked dirt), the knobs increase the pressure placed on the surface (less area for the same force), so there is a higher chance they will dig in versus a slick tire.
Some other factors:
- Having tread can somewhat protect the tire against punctures on hard surfaces as the tire is raised off the ground when the knobs aren't fully submersed.
- Knobby tires are arguably part of the look of a mountain bike, and riders might not like the association between slick tires and roadies.
- The knobs visibly wear much faster than a smooth tread, so tire manufacturers can sell more tires :D
Why are new bicycles sold with them at all if smooth tires are better nearly everywhere?
- slick tyres are terrible on anything except hard, smooth surfaces like asphalt
- most hybrid/mtb/commuter bikes will at least sometimes be ridden on gravel/dirt/mud
New road bikes are indeed sold with slick tyres.
I think a big reason for knobby tyres on hybrids and similar bikes is psychology.
People like to be prepared for everything (same reason why people buy 4WD cars, or huge cars in general, just to commute or buy groceries 99% of the time). Even if they basically never ride off-road or in snow.
I think many people don’t realize how little knobs help on fine gravel, which is probably the non-tarmac surface people are most likely to encounter (if they ever leave tarmac at all). Wide tyres with low pressure help much more on gravel than the presence of knobs.
People think they need knobs or treads, which is probably also the reason why manufacturers keep adding these negative treads which do absolutely nothing for grip (maybe they help to reduce weight?):
Edit: I should point out that for “real” off-road usage on rough gravel, tree roots, grass, mud or snow knobby tyres definitely help a lot. But I think most “off-road” tyres never see such terrain.