Licenses are a control and revenue method.
First you make something illegal.
"It shall be unlawful for any person to kill by any method any fur bearing or waterfowl animal within the borders of state x."
Then you make a set of regulations and a slip of paper that allows you to do something that is illegal.
The purchase and adherence to regulations that pertain of a hunting license shall be required to kill or mame any fur-bearing or waterfowl animal within the borders of state x.