When an aircraft is neither climb, nor field length limited, nor obstacle limited on take-off, there is essentially performance to spare. There is no point in taking off with the maximum take-off thrust set, when clearly this is unnecessary.

Any thrust setting, less than the maximum, will provide substantial benefits in terms of engine life, maintenance and operating costs.

In such cases, it is normal for the operators to use approved procedures to reduce the thrust, so that the aircraft is at or close to its performance limits at the reduced thrust setting.

Doing a take-off with less than full thrust does not save fuel. It actually increases the total fuel burn, because of the increased time to height. Nor does it necessarily reduces the aircraft noise. It is quite likely to leave a bigger noise footprint, because the aircraft is lower and closer to any noise monitoring site, than it would be under full thrust.

The purpose of the Reduced Thrust take-offs is to extend engine life and thus save money.