The APU provides power while the engine (turbine) is off, so it can lessen its heat signature while off. The SEP upgrade for the M1A2 has the APU under the armor, so its out of harms way, where earlier had it either in the bustle rack, or on the right rear. I also think that SEP gives a thermal management system to better the conditions inside for the crew and the electronics, keeping them within certain temp tolerances...