What is the headquarters good for?

The level of the headquarters building affect the number of rallied troop APCs you can have when you start a rally. To rally again Wendell the clown for example you need 3 in total. To do this you need to have the headquarters at level 4 or above to allow 2 other players to rally to you.

Headquarters at level 12 though will give you 4 slots which is the maximum, after this upgrading it is only useful for the additional power level it gives.