Just to add to what was said earlier, 'overhead' is usually referred to as 'headroom' in audio circles. Its main advantage is not to allow you to turn the output up as you get accustomed to the output potential of the system, its to keep your system from clipping even when those quick output spikes occur.
Remember guys, just because you set your gains with the amp not clipping does not mean the amp never clips. Music is transient, and the amplifier that is adjusted for maximum output for a given signal level will clip sporadically when a transient signal is applied (depending on the recording level, but speaking generally here). The fact that an amp with a lot of headroom will not clip even when the music calls for an unusually large output spike is headroom's claim to fame.