Price? Speed of service? Don't have to cook at home for those who don't like cooking or cleanup? Atmosphere? The company?
All of those might matter, though I would tend to bet toward the last two, atmosphere and company.
That's one thing a taproom might have going for it, even with "Meh" beer, i.e., atmosphere and company. I'm going to a morning coffee group today, bunch of guys get together for coffee at the Country Kitchen. The food there is pretty good, the coffee? Meh. But one doesn't go for the coffee. It's the company.
Interesting you mentioned this, because I don't think I've considered this part of it. The quality of the beer isn't all of it. Given two equal places in atmosphere and the social aspects, you'd expect the place with the better beer to win out. But if otherwise different, maybe the beer isn't the most important thing. (gasp!)
We have two basic beer places in my town (good beer, not the BMC stuff). One is better than the other in terms of the beer; the other has a better social draw, and the beer is good enough. I prefer the place my friends are gathering, despite the fact I can do better with beer at the other.