Priority — Optional. The priority of a particular URL relative to other pages on the same site. (…)
You can use this tag to increase the likelihood that your more important pages are present in a search index.
One way to spam a search engine is to create doorway pages meant to redistribute Google juice to the landing pages of your choice.
The method is efficient but has a drawback: The search engine must “find” pages that you typically do not want your users to find. Thus, unless you happen to use cloaking, you’ll use a few html tricks to make the links “visible” but not “obvious”.
But no longer. With the new Google Sitemaps feature, you can now serve your spam pages to Google directly, and give them a low priority on top of that to lower their chances of ending up higher than your preferred landing pages. Sweet, isn’t it? :)