For fish that come from more riverine locals a powerhead can be used to increase flow, creating a more natural environment for such fish.
In a planted tank we often find problematic 'dead spots' in our tank, where flow is restricted or even stagnant. Nutrients and CO2 therefor cannot get to any plants that are in these spots and this causes obvious problems for the plants natural growing potential. Also, fish waste can get stuck in these areas and create problems, so a powerhead in this instance can remove these areas.
Depending entirely on the type of set-up you have, will effect where you need your powerhead to sit