Guys, why did you agree?

personally if I did that it would be because I would rather be flattered and dance with some chick than dance with nobody and feel sorry for myself.

If he doesn't want to hurt her feelings then that is a very obvious reason that he would say yes. Even if she isn't exceptionally good looking, it's not a commitment to spend three minutes with your hands on someone's waist. I for one don't think dancing with someone is a big deal, and I'll dance with anyone who asks me so long as they aren't a creep or I'm supposed to dance with someone else. If this is puzzling to you, you probably like the guy. If you want to dance with him, go for it, or go talk to him, at least. He might want to dance with you just as much as you want to dance with him but feel awkward approaching you to ask.

There's a couple possibilities. He didn't want to hurt her feelings and say no. He's just a nice guy. He didn't have the nerve to ask anyone else to dance and got tired of hovering in the corner. Or he thinks the girl actually isn't so bad and might even like her a little. Beauty is the eye of the beholder. I guess you're asking because you like the guy. Best to just get over the dance (there will be others) and maybe ask or even keep this guy or another guy company next time.