A poster in a forum noticed a couple of years back that anything with the name sitemap.html or sitemap.php seems to get spidered more often than any other page (apart from the homepage). It may be that search engines specifically look for sitemap.xxx each time to see if any new pages are added.
http://www.highrankings.com/forum/index.php?showtopic=41893
Many in the SEO community argue that there is no greater from of SEO than forming an SEO Monopoly under niche keywords.
