Fat-Free-Framework global variables and functions

I'm new to fat free framework and i'm a little bit confused about the global variables.

$f3->route('GET /@page','display');

    function display($f3) {
        echo 'I cannot object to an object' . $f3->get('PARAMS.page');
    };

$f3->run();

Here i'm using GET /@page as a token for the url route. In the function i then use $f3->get('PARAMS.page') to get the value of that variable.

Since $f3->get is the method to get a global variable, why do i have to pass the $f3 class to the function.

The below code doesn't work ($f3 class not passed to the function).

$f3->route('GET /@page','display');

    function display() {
        echo 'I cannot object to an object' . $f3->get('PARAMS.page');
    };

$f3->run();

So my question is: why do i have to pass the $f3 class to the function?

Thx...