I'm pretty much the opposite of RawDog: a shrimp at 5'6, 150 lbs. As I'm really lean (ie: no ass whatsoever), I buy the smallest size made for men: 28. Chinos (khakis) and jeans stretch out very quickly, so even if they fit correctly just after they've been washed, an evening out (especially when weighted down with the usual wallet, keys, lighter, phone, cigs, etc) is unthinkable without a belt, as I'd be holding them up within an hour or two :redface:
For me, belts are mostly a utility feature. However, there is an undeniable fashion aspect. Remember that men have far fewer options to accessorize effectively without appearing ridiculous compared to women, and most menswear is conservative by design (limited color and cut options available). Belts give men a little something to play with and have some fun wearing, though I'm glad that the days of the ginormous, gaudy buckle have passed; it was a really
douchebaggy look.