There are loads of rackmount power amps, but if you want a compact lightweight rig there are fewer options at a decent wattage. They're not significantly cheaper than an equivalent "normal" amp that can be run flat (either via the EQ, or the FX return) either. Also, if you've already done all your tone shaping on the pedalboard and sent your signal to FOH, then having EQ controls on the amp lets you tweak the sound on stage independently - e.g. to compensate for having to using someone else's cab, or being shoved in a corner, a boomy stage, etc.