We don't use the take-off config test when preparing the aircraft on the ramp before pushback.
We use it just before we line up on the runway. It is to simulate the application of take-off engine power, and if any monitored system is not set correctly, a master warning will be triggered. This test allows us to remedy the mis-selection before we actually apply take-off power.
Flap setting is one parameter which is tested in this way. If the actual flap setting is not the same as that which we have programmed in the take-off performance page of the FMGS the config warning will be triggered.