Miserlou,
they don't.
Whatever you do, Attitude + Power = Performance; if you change either attitude or power you will need to change the other inversely to maintain the performance. Which you do first, is a matter of choice however, instrument lag and aircraft inertia may determine that one way is more effective than the other in certain circumstances.
When climbing and descening at a fixed power then attitude is the only way to control airspeed, which of course should always be trimmed, if you instructor tells you otherwise change him for a better one.
The object on an approach is to fly a line of constant bearing from the point where you roll out on final to the touchdown point. Personally, I find it much easier to hold that bearing optically than fishing arround with the attitude and descending in a series of steps. If the attitude and power remain constant only the met will change things.