[NTLDR]

Its convenient because variables are global as the name sugests. For example it may be posible to overwrite a value by appending it to the URL:

EG: URL: test.php?debug=1

PHP Code:

if ($debug)
  // do stuff
} 


In the above example it checks if $debug is set, if there has been no declaration within the file and register_globals is ON then it will take the variable value from the URL, which may not be desired. If register_globals is OFF then you would need to use $_REQUEST['debug'] or $_GET['debug'] to access that variable within the script.

vB2 works with register_globals ON or OFF and registers the globals for you if its set to OFF. Its convenient because many scripts still require register_globals set to ON, hence why many hosts have it turned on.