Function differences in return and echo

Question

Why use return when echo does the same as return?
Example. Here is a function that returns the res.

function mult($num1, $num2) {
$res = $num1 * $num2;
return $res;
}
Mult(3,5);

And here is code that echos the res.

function mult($num1, $num2) {
$res = $num1 * $num2;
Echo $res;
}
Mult(3,5);

The answer will be same. Same results. Right? So can someone help me understand what and when and how and why to use return.