I buy downloadable games because anymore they can offer just as much fun and replayability as a AAA title can.  They are less expensive and I can circumvent dealing with any dumbfounded employee that is all too happy to tell me about their latest card or reserve service.  
 
Fat Princess, Trine and Shadow Complex are 3 recent games that demonstrate there are some very high-quality downloadable games out there that have excellent presentation and offer a lot for their price.  More and more come to the market each year and already I feel saturated, but the downloadable games generally offer new ideas too that a AAA title would be too afraid to do.  Downloadable games allow developers more creative freedom with less risk involved.