It's a programmer for the fuel injection, It comes in many flavors; PowerCommander 3 and 5 and a variety of dobeck units (FMF, EHS, HMF, Two Brothers etc.etc.) It is a part of the Big 3 that everyone refers too. Any of these units are meant to adjust the fuel curve much the same way a rejet works for a carburator.
The reason for one would be to compliment and accentuate the addition of an aftermarket exhaust and free flowing air filter, the more air that flows through the engine the more fuel can be burned for more power. Some like myself add one to compensate for the lean condition from the factory, plus when I'm ready for an exhaust I'll be ready.