A function doesn't have to return something for it to be useful. Most of the time when you want a function to return a value it's so you can work out whether whatever the function was supposed to do was a "success" or "failure".
You could have a function, for example, which just increments a counter that is stored globally (I'm over-simplifying the concept of passing variables by reference, etc).. that function doesn't need to return anything to the part of code that calls it.
