I can't speak to why anyone else would want to do it. In my case, I have a massive collection of digitized music that I prefer to the radio or cassette player. It's trivial to put that music on a deck-of-cards-sized computer and hook it to an amp connected to the car speakers. Mounting the amp and computer is a little fiddly however. The easiest way for me to mount that in the car would seem to be throwing it in a $10 drawer.