The signal processor controls the frequency range delivered to each amp. It's a electronic signal processor/crossover. I have 4 amps and 5 pairs of component speakers not counting the subs. Each pair of speakers has it's own amp or one half of a 4 channel amp. So I am able to dial in each speaker. I have 12" subs, 9" mid base, 6.5" and 4" mids, there are frequencies that all 4 pairs of speakers can play, therefore without a signal processor some frequencies will be over powering. Bottom line I am able to really dial in my system and adjust for accustics of my car.
The down side is between the EQ, the amp crossover, and the signal processor there are countless mini adjustments that can be made and you can become obsessed with trying to make it "perfect".
