jQuery form. Why to use it?

jQuery form. Why to use it?

I wanted to ask, why is it good or better to have a script prinf the form
then plain html. And what's the difference? When I type:
<form><input type="button" value="button" /></form>
or
<script type="text/javascript">
$form = $("<form></form>");
$form.append('<input type="button" value="button" />');
$('body').append($form);
</script>
What if user disables javascript in his browser?
P.S.: Should I use javascript to build and validate a registration form,
or is the plain html and php enough. For security of course.