What is the cite attribute for?

The cite attribute specifies the address of the source of the quoted text, I think, but who uses this information?

For example:

<q cite="http://www.example.com/quote">
<p>&#8220;A quote&#8221;</p>
<footer>&#151;Person quoted</footer>
</q>

The source of the quoted text isn't visible to the end-user in a normal browser, so who does use this information, and how?