1, adds to the overall turnover of the tank and helps keep water quality as good as possible
2, always makes sure that you have one mature main filter that is running when the other one is cleaned which again helps water quality
Your main aim is to keep good water if you can do that you should be able to keep good fish

Some powerheads can be turned down but ideally you want them running at maximum, Martin has written a good article on powerheads - http://www.loaches.com/articles/water-m ... h-aquarium