Links to product searches are often added to sitemaps because they're typically dynamically generated during a search (like on eBay) and usually contain a session ID too.

By providing a crawl-able link to products with keyword included, the search engine bot can index them because they can't perform searches themselves. The only other means to discover dynamic content is to create [Snapshots][1] of them.

Sites that rely upon AJAX and database driven content typically do the same.

Sitemap [size][2] is not an issue, providing that each one is under 50,000 links and less than 10MB in (compressed gzip) size. It will just take longer to crawl and index your site, and take up more resources in the process. This is done just to help the search bot index the product search based on keywords.


  [1]: http://support.google.com/webmasters/bin/answer.py?hl=en&answer=174992
  [2]: http://en.wikipedia.org/wiki/Sitemaps#Sitemap_limits