It means that the military works for the king and not for the people that it should protect. He refers to the King of Britain and the sentence means that he basically uses the army not to protect the people, but as his own personal army to fight wars against who he thinks is the enemy. Instead of military working to protect the people, the military does not have accountability with the people.
It was important to write it down in order to have an official agreement they could preserve. They also needed to make a statement towards the British about the separation of the colonies from the British Empire. It was a formal declaration towards the King and that they weren't willing to back down.
