Android : If onDestroy() does not clear all the variables, what is the purpose of onSaveInstanceState()

In a course, I learned that onSaveInstanceState() is necessary to get the variable values before onDestroy() is called, but according to What exactly does onDestroy() destroy? the variables are not actually cleared.

So my question is, what is the use of the onSaveInstanceState(), if the variables are saved anyways?