I don't think this line is required to make foo.php?key=value to work, because the query parameters aren't treated as part of the path. It's for URLs like foo.p

I don't think this line is required to make foo.php?key=value to work, because the query parameters aren't treated as part of the path. It's for URLs like foo.php/bar where you have some path after the PHP file that's passed to the script - this was supported out the box by Apache mod_php and there's a fair amount of code out there that relies on it. (Generally because cleaner methods of feeding a set of paths to one PHP script require server configuration that may not be available on cheap shared web hosting.)