If a search engine stops downloading sitemaps then the admin knows something is wrong.
Its also very useful to see which users downloaded the sitemaps. Not only Bing and Googlebot, but other bots will follow. Some genuine search engines (of which its nice to know they are indexing the site), others commercial services (which the webmaster may or may not want to block) and also malicious bots like scrapers. Its useful to keep track of such activity.