Piro comp is great for keeping the right amount of forward trim to make great pirouettes in fast forward flight. It knows that your pitch "I term" wound up a little to hold that angle and when you yaw around, it transfers the windup to (say) left roll, then reverse pitch, then right roll, then back to forward pitch. This makes it hold the bank angle correctly.
Piro comp is horrible if all you want to do is pirouette when you hover, but your (multi, heli) copter is not perfectly balanced, or for instance on a quad, one of your motors or props is not matched to the others. In that case, you want the windup to stay on the motor it started on, not move to another motor.