Let's say you are cruising from point A to B and you have a fairly stiff cross wind blowing the entire trip. The wind will make your boat lean toward the wind and the trim tabs will help level the boat back up. Same if you have a lot of weight on one side of the boat, you can counter that with the tabs. You use your engine for front to back trim and the tabs for side to side adjustment.
I'm sure there must be a guide on the Bennett site to explain this better than I am.