What is the advantage of 950 preamp/7100 amp combination over the Denon AVR-4802 that cost almost the same?

Among other things, the 950 has bass management on the 5.1 analog inputs. The Denon doesn't.

Any time you separate the high voltages and currents of the power amplifier channels from the more sensitive preamp/processing functions, I believe that you get a benefit in sound quality. Not that you can't do this just as well in a receiver, just easier with separates.