PHP call_user_func vs. just calling function

I'm sure there's a very easy explanation for this. What is the difference between this:

function barber($type){
    echo "You wanted a $type haircut, no problem\n";
}
call_user_func('barber', "mushroom");
call_user_func('barber', "shave");

... and this (and what are the benefits?):

function barber($type){
    echo "You wanted a $type haircut, no problem\n";
}
barber('mushroom');
barber('shave');