When you are trimmed straight and level at say a lowish power setting and you advance the power levers fully forward what happens that causes the aircraft to need trimming to maintain the altitude?
The aircraft then climbs because the engine power is being translated into extra potential energy. The engine power has to go somewhere, and it cannot go into an increased speed because the aircraft prevents that (ignoring cases where pitch attitude is very power sensitive because the thrust is excessively out of line with the hull).