My guess is so they don’t slip out of your hand after the condensation makes them wet. I don’t think it’s so they fly like a golf ball

It might also be a form of decoration. Of the beer mugs that I have, I prefer the dimpled one to the smooth one.
Or a weight- or glass-saving measure.

Or totally for making them fly like golf balls. :D

Some history here suggests it’s aesthetics.

…or some aerodynamic thing that makes you drink faster so you’ll drink more? I dunno. That’s an alternative answer in case you don’t jive with the others. But I would love to see a beer glass fly like a golf ball.