There's no possible situation I can think of except for a record speed attempt for using this switch, it's baffling that it exists. Even if it increased engine thrust by 10% somehow, compared to the 80% boost from reheat it's a trivial addition. And if there were any serious combat scenarios in which a 2% RPM boost at the turbine for a few minutes would mean the difference between victory and defeat I would think they would have redesigned the engine to make that available ALL the time. I'm very surprised it wasn't just removed from the aircraft.