Derate vs flex
Derated thrust is used because it programs the engine in such a way that you achieve the derated amount of thrust when pushing the thrust levers fully forward (when using assumed temperature it is possible to advance thrust beyond flex thrust to TOGA thrust). The derate defines the new TOGA setting. This allows you to decrease Vmca and related speeds, due to the lower yaw tendency at an engine failure which can be quite useful in some scenarios...
At some aircraft even a combination of derate and assumed temperature is possible.