Take my "Burst" suit for example.
It's armed with twin linked burst cannons, and a missile pod (in the shoulders). However, it only has three hard points. So, if i field it as a normal suit, those take up all three hard points and it can't fire them all in one turn.
However. If I upgrade it to a squad leader, it gains access to the armoury and can then take a hardwired multi-tracker so it can fire two weapons in one turn, despite already having it's hard points filled.