It might be useful for programmers in other languages, so they don't have to either come up with english names, less descriptive names ("id1"), transliterating words, or replacing letters with "similar" ones ("O" or "OE" for "Ø")
Funnily enough, I just registered here, trying to use my first name Øystein, and what did ol' hackernews tell me? "Usernames can only contain letters, digits, dashes and underscores"