Sweet jesus, this is probably the most overlooked/misunderstood features of forms even today, and it's basic HTML 4 or possibly earlier.
It binds the label to it form element, most importantly: IT FOCUSES THE ELEMENT WHEN YOU CLICK THE LABEL.
<input id="foo" type="text">
<input id="foo" type="checkbox">
IMO, you cannot call yourself a competent web developer if you fail to understand and implement this consistently. You'd be amazed at how many hugely trafficked sites fail to do this.