The whole idea is to get the EQ on the amp sounding as rich and full as possible. Some people have commented on the way I dial in an amp. When Scott F came to my rehearsal studio, he thought it was funny how I quickly jump around the knobs, twiddling them real fast to find their sweet spot. I almost do it without looking, just listening, then I usually notice that the knobs are a little different than I might have them set, just by looking.
Mids give all the growl. Without them, you've just got a big wash of white noise.
Anytime I see a band with that "scooped" Rectifier, Triple X, or Valvestate tone, I almost start dry heaving. Those are the amps that NEED mids, so it's crazy to turn down the midrange. You can always tell when a player needs mids, when they go to a solo tone and it sounds like a barrage of high pitched noises peeking out from the rhythm section.