But if you want to find a feed for your favorite site, you may not know what the feed address is. Or you may know they have multiple feeds (either for topics, or to support every flavor of RSS and atom) and not want to have to test every one of them or risk creating a duplicate. Being able to put either a keyword you think will appear in the title, or in the site's URL, would let you find all the feeds from the site.

I don't know if it would be useful enough, but I can see where just using the feed URL would be insufficient in some cases (I've encountered them before, and carried on wondering if I just created a duplicate feed or not).