[jQuery] fn question

I found these both work, so what is return for?
<script>
$.fn.btnClick = function() {
    this.click(function() {
        alert('test');
    });
}
$('input#btn').btnClick();
</script>
<script>
$.fn.btnClick = function() {
    return this.click(function() {
        alert('test');
    });
}
$('input#btn').btnClick();
</script>