What most of the bands are getting - apart from a tame night's beer money - are a few more hundred ears, and the chance that a few people between those ears will love a song enough to seek out the band's music and spread the word.
It's a chance for a handful of people to get really involved in your music. Learning the words enough to sing them, learning the parts enough to play a chart. Sharing the song with friends to play along. That kind of intense interaction with a song is almost impossible with an mp3 that you download and forget on your hard drive until it comes up in a shuffle, "What the heck? When did I buy THAT?"
It's not about what music is worth. If this were about artists getting a fair price for their music, there wouldn't be people here fussing about $1 or $2 or $3 songs. I know what my songs are worth to me, and believe me, you can't afford them. But I want people to hear them, so I share them. For free, mostly, or in this case, for dirt cheap.