Ok, I think I have grasped what needs to be in the explanation ....
Consider an aircraft in a steady unaccelerated climb or descent ....
1. The vertical component of lift MUST balance weight or the aircraft would be accelerating upwards or downwards.
2. Further, if the aircraft is CLIMBING, the thing that makes it climb is excess THRUST. If it is descending, then you may reduce thrust.
3. If you are climbing, then the flightpath, being angled upwards, yields some rearward component of lift that must be overcome by thrust also.