Well, the only things I can think of is so you can run more speakers off of the extra amp (ie: speakers in another room or outside).
Or, if it's an old prologic amp, I've heard of people running the rear channels through it to extract the rear center info for a poor man's Dolby Digital EX set up.
Also, the extra amp could be used to power an external subwoofer by running the LFE output through it. But, most retail subs come with their own amps, so this would be more of a DIY Sub solution.
Most seperate outboard amps sound better than amps built into a receiver.
Better power supply, better capacitor, more wattage.
Look at a speaker like a Paradigm Studio 100. More than few people have said that this speaker sounds WAY better with a better amp. 100 watts a channel isn't enough.
Try running Martin Logans with a receiver ..... Couldn't do them justice!