It makes it easy to crawl. If you're trying to get all of the user profiles for a given website and all of the URLs look like foo.com/user/123 it's easy to down

It makes it easy to crawl. If you're trying to get all of the user profiles for a given website and all of the URLs look like foo.com/user/123 it's easy to download all of them, whereas foo.com/user/1234-abcdef1234-1234 is much harder to guess.