From what i've managed to pick up, it depends on a few things. Personal aesthetics can be one, but on a more practical level it's good for using the radio in-car (much more difficult in most instances to have an FM tuner inside your PC) and also for people who hook their PC sound to the car through an FM transmitter.
As to specifically have a CD one, i'm not sure. I'm personally wondering if I'll end up ditching my pioneer mp3 after a carputer install and go for a good simple radio.
There may be better reasons than mine though