OPML sitemaps pre-date the blogosphere which is where they have the greatest popularity. Websites such as PubSub.com, AllRSS.com, and FeedFarm.com all accept the OPML format. The main site for all things OPML is at
http://www.opml.org which may give you a better insight into it's history and uses.
The long and short of it is that it gets passed around on services much like RSS feeds do and that it is probably best-suited for blog and article-based websites. I personally use my OPML sitemap in my
blogroll so that my navigation from my blog to the site is easier.