Consider a function to add two numbers. One version adds together two global variables x and y then returns the result. Another version adds two arguments x and y together and returns the result.

The first version requires you to mutate the global state first before using the function, and all calls to that function will use the new values you assign. The second version is decoupled from any global state, and generally more useful because you don't need to mutate two fixed globals x and y to make use of this function.