You're right - there's an awful lot of overlap between the gear ratios offered.
I dunno about your riding, but I have a 21 speed (3x7) and the lowest gear is 26/28 (that's a 26 tooth front chainring with a 28 tooth big cog on the back block) and I occasionally want one lower on the super steep climbs.
At the other end I have a 52/12 and on a downhill I am spinning out too soon.
This shows that 28/16 is about exactly the same ratio as 48/28.
You might choose to be on the big chainring if you're about to go down hill or flat and smooth, but if the next section is an uphill you would want to be in the little chainring ready for the grade.
So the middle chainring is useless? No - without that the front mech would have to lift the chain from the small to the big chainring, and that's a gap of 20 teeth. Most derailleurs can cope with 13 teeth gap, not 20 teeth.
To make best use of your gears, you want to ride at a pedal cadence of ~80 RPM. So pick a gear that lets you spin the pedals at about this speed. Too slow and it hurts your knees.
Many riders live in the middle ring and use the rear mech to get gears, only changing front ring in the extreme of headwind/uphill or downhill/tailwind. Personally I live in the big ring all the time, until it gets steep.
Changing gears will also help spread your overall wear, and lets you get moving quicker from a stop at the red light.
Edit: Here's a segment where 26/28 is only just sufficient: https://www.strava.com/segments/9230838 Strava says the grade is in the upper 30's percentage, and having ridden it I'd agree. The main problem is keeping the front wheel on the ground.