The keyword feed is probably the most powerful type of feeds offered by SSP.
What a keyword feed does, is that it indexes pages generatred through the internal search engine of the website.
For instance, let's say you're Old Navy. You have a search box on your site, which generates results pages according to the suer query. So, if I'm on Old Navy.com and type "Green Skirt" in the search box, i'm prompted to this page: http://oldnavy.gap.com/browse/search.do?searchText=green+skirt
This page does not "physically" exist, as it is generated by the website "itself" when a visitor submits a query in the search box. No bot, not even GoogleBot, can type queries in search boxes. The generated pages are hence totally invisible and inesitant as far as the SE bots are concerned.
What SSP allows you to do, is submit these urls for indexing. Pretty cool huh? Imagine the added benefit for Old Navy... they could have a crystal clear snippet on the Yahoo SRERP for long tail queries such as Purple Shirt with Sequins (http://oldnavy.gap.com/browse/search.do?searchText=pruple+shirt+with+sequins).
And thanks to the smart engine, even if there is no such item, there will still be a relevant result page to be served to the user.
As one of the SSP guidelines for this feed is to ensure that there is at least 1 product proposed per result page, it ensures that no visitor would be redirected to a customised 404, or even worse, an plain error page.
This is probably the most powerful tool in the way that one can easily imagine the amount of new urls one site can get indexed. I remember one client of mine, very similar to Old Navy, for which i boosted the indexed pages from 15000+ to over 800K!I dont even need to specify the boost in traffic that resulted ffrom that, do I? :)
It can also be very interesting for travel sites, like Expedia, as it can "pre fill" forms, and hence serve real informational pages on travel queries such as "flight from new york to tulsa". Wwe could imagine that we'd have submitted such a url, and the web user would then land on the expedia flight page, with new York already typed in, in the departure city box, and Tulsa already entered in the arrival city, and then logically, already the results of the query.
This eliminates several steps to the act of purchase, and brings customers that much closer to converting!
More entry popints to convert, fewer steps til the conversion tunnel's end --> More business for you!