I've never used them... really necessary?
Everything I've seen and read, they're for current and water circulation more than anything else.
So if you were only running one HOB filter for example on a long tank, you could use a powerhead to get the crud back to the other end where it would be sucked up instead of settling in the far corner. If you had a canister, it would more or less make the powerhead unnecessary, depending on the size, style, and power of the canister - because it would make a pretty good current on its own and you could put the intake and outtake further apart than with a HOB.
Or, if you wanted to do a river setup with lots of current for fish that like cool flowing water, a powerhead would work great. You can get a sponge to put on it for some basic filtration but I would generally have some other beefier filter alongside it.
