the why is probably because I like amps.
if run together, my total amps and cabs is comparable to what the who, cream, or zep were running when I saw them - and running biamped or tri-amped is fun and sounds good. one can wish for the opportunity to utilize it all one day.
but what I've really got is versatility - tube amps with 34's, 6550's and 88's, and SS. the biggest is a 200, and I could run that through one 10, or pull a pair of 34's and have 50 watts in the 10, or run 16 ohms with the SS.
the Sound City 200 (kt88's) through one or two 15's is my standard/favorite as for most stuff I prefer that sound, for guitar or bass. but I switch it all around for variety.