basically you can put a whole amp setup in a rack for efficiancy, choice(you can always swap one piece out, like a preamp..etc), and you can often get better sounds for less, a used pre/power is gonna be cheaper usually than a used 2000$ head and you could equal the sound..
each rack piece usually has it's own power cable
here's the rack i want to put together at the moment
(6 spaces)
Gmajor effects-1 space
midi switcher(ground control) so i can control everything from the one unit- 1 space
egnater IE4 preamp- 2 spaces
Peavey classic 50/50- 2 spaces
it means you don't have to worry about carrying, controlling, or seting up heaps of pedals, and you have great freedom with preamp and poweramp choice