because artists make no money. no money at all. it's a nice way for artists to make additional revenue that they desperately need.
plus, it's a great branding play. it helps artists establish personal connections with their fans that go outside the standard concert experience. it sets itself apart from kickstarter as it doesn't tie these experiences to large scale projects (i.e. albums, dvds, etc)
this is an amazing idea, go bandpage.
There are plenty of other ways for artists to make additional revenue. If an artist wants to "establish a personal connection with fans that go outside concert experience" they can set up meet and greets or go out back after shows and set up web chats for free. Charging anyone excessive amounts of money to meet you or for a bit of your time isn't a "personal experience" at all.