Why do not declare report variable as global?

A major reason for local variables is that you get a new one each time you call the function. This enables the concept of recursion, where a function might call itself (with modified arguments) to do a portion of the overall task. Each call has a "private version" of the variable to use without altering the one in the caller. If only global variables were used, recursion would not be possible because all calls would share the same variable.

Another reason for keeping variables local is to make the code easier to maintain. Global variables can be modified by inline code or other functions, but local ones cannot. This limitation is called variable scope. In a perfect program (where recursion is not used), global or local would not make a difference. But the more global variables are used when local ones will do, the more opportunities there are to have problems later.