I unlocked the ability to upgrade a Command Post which I purchased.
Was wondering, what purpose does Upgrading the Command post give.

thanks

Gives you and your team additional supplies or health at the command post that you capture. Note that you have to manually upgrade the command post by accessing a captured command post and waiting for the upgrade wheel to fill up.

Hot-Wire has it right.

More specifically, instead of giving you only 1 extra pip of Health or Supplies (depending which type of CP it is), upgraded CPs give you 2 additional pips - which can be a huge bonus - an extra buff on yourself or a teammate, or enough extra health form the CP to make your Light tougher than a Heavy? Who'd turn that down?