De facto:
Think about the days without FMC's and VNAV.
The engines do not know you have it aboard anyway.
Even less they are caring wether you are in climb, cruise, descent or approach.
However, there is a Maximum Continous Trust they are rated for, in short: MCT. You can use this thrust setting until your next oil change.
At the point you will need MCT to maintain cruise altitude, you reached your maximum altitude for that speed.
As this is not an advisable operating, you fly lower.
Based on margin calculated, you will get a lower, Normal, 'max' Cruise setting, below the engines will operate for given weight, speed and ambient conditions.
This setting will also prevent the thrust levers going "al the way"( to what?), just to adapt for normal speed deviations, in which there is in fact no "hurry".
The FMC will do that for you.
Thrust this makes some sense.