There is no real purpose, from everything I can tell. It's just one of those confusing Microsoft things that is not explained well in their documentation, and has to do with the underlying way they implemented subscription creation. The only time you will care about it is if you are programmatically creating subscriptions (in which case you need to maintain uniqueness of the aliasname).

“Theirs not to reason why, theirs but to do and die” -Alfred, Lord Tennyson