They can work like `const` variables, except you should use each where it makes more sense. For example if you want to return the screen's width (for example) and you forced your game to only have 300px width and nothing else, you could `return 300;`, but your code would still call `getGameWidth();` to get that value.
So in the future, if you decide your game can support an arbitrary width, you can change the function to return that instead, and the code of your game doesn't have to change at all.