So far we used to make use of a property-set method to call a function. Since the property-set method is used, we have to make sure that the function that we are using must return a value. In most of the cases when we write a custom function for data processing, we will end up not having a return value, simple the function returns a void. But then we don't have a way to refer to such functions through a property-set. I see this method is particularly used in such cases since the return value holder is optional. This can come very handy when we are supposed to use a function that doesn't naturally return a value.