One reason to do so is to be able to turn your TV/cable volume up only when you want to hear what is on the screen and otherwise listen to a CD/Music/etc. in the background while occasionally glancing at the TV.
I used to do this often while reading before getting a projector and I miss being able to have one video source running and another audio source. The Denon 3808 allows for this to some extent, but it won't let you do this with any HDMI sources, which limits the functionality quite a bit given that most of what I have is plugged in via HDMI.
Jason
