Is it important to use <label> in HTML?

Question

Excuse me for the maybe very stupid question, but my curiosity is killing me, and plus I'm new to this, but are using labels important in your markup, and why?

i.e.

<label for="birthdate">Birthdate:</label>
<input type="text" id="birthdate" name="birthdate" />

Why must I label this input, why is it beneficial to the user, why is going to be beneficial for future use, or is it search engine optimization thing. That's what I really wanna know :)

Answer 1

It's important for a number of reasons:

• Clicking the label focuses on the
  text field, which is something a lot of users expect.
• It's helpful for the accessibility reasons.
• How else is the user going to know which field is which? You could use just text or a span or something, but why would you?
• It leads to a more semantic markup.

Answer 2

It helps for accessibility, e.g. screen-readers.

Also for things like checkboxes it allows the user to click on the label and not just the checkbox itself (Try it!).