Because if you use:
$name
$email
$submit
$othervalue1
$othervalue2
instead of
$POST['name']
$POST['email']
$POST['submit']
$GET['othervalue1']
$_GET['othervalue2']
then I can just do this:
http://www.test.com/yourscript.php?name=elvis&email=elvis@slashdot.org
&submit=true&othervalue1=9102&othervalue2=5930
In short, $name, $email, and $submit do not discriminate between sources. $POST will only accept a POST method, $GET will only accept a get method.