They're for rack mounted guitar pre-amps that don't have thier own power section. They go in racks as well.
DJ's use them for PA's and stuff too.
more strictly a power amp takes a line level signal and amplifies it to a signal ready to be sent to speakers.

whereas a preamp takes a passive signal and amplifies it to a line level signal.
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."