Why are "for" attributes for form labels necessary?

Question

Why are "for" attributes for form labels necessary? I've never had a use for them

Answer 1

The main advantage is that clicking on a label with a "for" attribute will auto-focus on that form element. So, a label for an input field will be associated with that input field, and clicking on the label will autofocus the input.

Answer 2

@ChristopherArmstrong's answer is technically correct - but the reason it's a good thing is that people who have trouble pointing correctly (older users, people with disabilities, etc.) are helped by this. It lets them get the cursor "about right" and still land in the right field.