Giving you a place to try them out. Giving you instant gratification. Giving you things you may need in an emergency situation. There are still quite a lot of reasons IMHO. OTOH, that's no reason you shouldn't go internet or direct if you're so inclined.
Nail on the head. When I first started my job here at Vigier, I pushed for us to sell direct. As a small manufacturer in another country, we always have problems of scarcity, but selling direct wouldn't fix that. I'm working on building up our dealer network so someone in Des Moines can try a bass before they buy it.
Also, I did retail for almost a decade, I took this job to get out of it. It's a LOT of work dealing with customers. Dealing with supply side stuff is also a LOT of work. I don't want to do both; I couldn't do them equally well if I did. Of course, dealers don't want manufacturers selling direct and it really is an ultimatum in a lot of cases. We'll see how the Fender Direct program works, but I know a lot of heavy hitter dealers are turning their backs on a brand that was their bread and butter for a long long time.
Back to OP - you gotta do research. I buy/sell/trade a decent amount of gear... a lot is luck (magic $200 8x10, for instance), but a lot of it is knowing what I want, knowing what it's worth and monitoring the right sites to find it. On the opposite side, I know that a lot of times I paid below market value for something, so I don't mind letting it go for below market value (which makes it sell faster). What's the saying? Success is where luck and preparation intersect? That applies to buying and selling used gear.
It also helps to know your local store. My buddies work at a store about an hour from my house. I went in, noticed they had WAYYYY too many bass cabs and used that to make a trade that I felt was totally fair for both sides (I got a really clean Big Ben and an equally clean Henry the 8x8 for a somewhat beaten Megoliath that I paid $325 for). They were happy to unload a couple esoteric cabs in exchange for a more 'sellable' one of perceived higher value and get some floor space back.
Normally, the only way you can make out in a store is to trade in something for something else. As was mentioned in this thread, you can't give people what their stuff is worth in cash and just turn it around. The guys on the floor need to eat, the lights need to stay on, the taxes need to be paid. Like the quote from Jimmy above, you're paying a premium for the service of having somewhere to go where there's a lot of stuff for you to try.