Trim Tabs provide stern lift to help boats plane, and help them plane at lower throttle settings. They can be real effective on stern-heavy boats like ones powered by sterndrives. They will help all boats plane to some extent, but are less effective on boats that are not stern-heavy.
I had a set of Bennett Trim Tabs on my 19 SeaRay I/O. They worked real well for me. I do not have those on my 21' outboard boat, as the stern is a bit lighter than the bow. That boat has no issue getting on plane.