According to the MSDN documentation for Finding the Metabase Path when Given the URL, such a facility might be useful because "When IIS servers host a lot of Web sites, a Web site adminstrator might not remember the metabase path of each site or virtual directory."
Clearly, since I can't remember the metabase path of even the site root of my default website, let alone of every image file on my web server, I am not a real web administrator.