Some (not all) Boeing aircraft will, by the use of TOGA, automatically set the calculated EPR from the FMC for take-off.
This saves the PNF manually setting it himself. All he has to do then is visually check that the power has been set on the engine parameter display.
If I remember you can engage the autothrottle on flight sim, hit the TOGA switch and the engines will spool up to TOGA power without overstressing the engines......try it.
The only other time you'd use the switch would be in a go-around situation, where you'd require max power instantly, leaving you to concentrate on flying the aircraft without worrying (initially) about power settings.
Hope this helps