Returned values can be ignored

Functions can be designed to either

When a function does return a value, the program that called the function can either

In some cases where a function performs an action and also returns a value, the calling program may elect to ignore the returned value.

If the sole purpose of a function is to return a value, it wouldn't make much sense for a program to call that function and then ignore the value that is returned (although that would be technically possible).