Trim, IME, is very poorly taught.
I was never taught what the trim does in my PPL training. It was explained that it is used to remove the back pressure, which is true but beside the point. Actually, it sets the speed at which the plane wants to fly. The engine is nothing to do with that.
So if you are trimmed for 50kt, and are flying around at 70kt, and you get distracted, the speed will fall to 50kt (it will fall more actually, due to the phugoid behaviour) and you will plummet.
Correct trimming (for the desired speed) in all phases of flight is absolutely key to safety.