Decoy Commander is simply just a fake commander. It can build most of the units and buildings, including mines.
It serves the function of confusing your opponent. It is reasonable to build it when you have your last commander, so the enemy might get tricked and focus fire on the fake one. Its D-gun doesn't deal much damage.
The Decoy Commander has a lot of buildpower and is very good to assist in late-game mass-production.