php - Whit what is yeild different from echo

What is the difference between them and this two examples:

function result($r){
    for($i=0; $i<$r; $i++){
        yield $i;
    }
}

foreach(result(15) as $r){

    echo $r;
};

And one with echo only:

function result($r){
    for($i=0; $i<$r; $i++){
        echo $i;
    }

}

result(15);

I am sure there is some but what is it? Couldn't answer myself after some googling, sorry.