Here's a few reasons for thrust changes:
1: The aircraft may be in a lower class of airspace or traffic ahead and therefore may have a speed rectriction. This means that you do not need as much thrust to maintain a sensible rate of climb at the same speed. You would not enjoy sitting at the back looking up at a really high nose attitude and granny may s..t a brick. Think of your car going up a hill....you don't need to thrash the guts out to go 40mph.
2. As above in the last post, Noise is a major player in keeping granny in her garden happy and off that phone to ATC. Thrust is reduced to limit noise and air polution.
3: Engine management. With the cost of an engine these days, airlines cannot maintain poor economics. Would you thrash your car everywhere to the red line and trust it driving to France with the kids?
It would cost a fortune and probably go bang!
4: The thrust may be reduced when the aircraft has passed a strong inversion (turbulant or windshear layer due to a marked temp change) and a reduced setting can be used, again for engine life, noise and comfort.
5: If taking off with a tailwind (perfectly safe in the limits) then thrust may be reduced when at a safe altitude.
I think that this is the simplest that any of us can put it without any tech. input.