Why use SERVER PHP SELF instead of

Question

In a form on a PHP page, you can use:

<form action="<?php echo $_SERVER['PHP_SELF']; ?>" ...>

or

<form action="#" ...>

or

<form action="" ...>

In the action attribute of the form. Since echo $_SERVER['PHP_SELF'] does not pass variables for using GET and you have to use "", why would you use that or "#"?