You know what, I have absolutely no idea. It appears that it's a feature designed to run queries after the page has rendered. All the queries in shutdown queries are not run at that particular part of the script I can tell you that. If you look at exec_shut_down() in functions.php you'll see what happens. However, I can't find any reference to that function being run anywhere in any of the vB files
Who knows... maybe I'm overlooking something