Source : Yahoo Search Blog
The two most important uses I can see immediately are being able to tell Yahoo! to strip off affiliate ID's and Session ID's, so as to not only avoid duplicate content but also to merge link popularity more quickly and efficiently. Apparently this will also help them to crawl your site more thoroughly. Beyond that it would also be an easy way for webmasters to see if they have a duplicate content issue being created by certain query strings.
Hopefully Google will offer something similar before too long. It's a good idea IMHO.