Thanks for the replies. The obvious answer is regarding sources and level control. I wondered about people who use it to color the sound, as some salespeople have said "it just sounds _better_ with the ($5-15k) preamp!" and based on the gear they were talking about, it seemed kinda BS. But I know there are power amps which the designer biased on the low or high side.
I think if I needed input selection or attenuation, I would try to build or look into buying a passive pre-amp...