Lower thrust settings increase engine life and reduce maintenance costs.
Since the turbine limit cannot be exceeded, any increase in ambient temperature results in less thrust.
The flex temp procedure recognizes that an ambient temperature beyond the flat rating produces less thrust.
[3] The flex temp is entered into the flight management system (FMS) — effectively telling the engine control system to assume the specified air temperature instead of the actual outside air temperature in calculating the maximum thrust available under current conditions.
Advancing the throttle levers all the way to the take off/go around (TOGA) detent can still be used to request full rated thrust under actual conditions.