Does "\root" do anything in /etc/aliases?

 but I can't find what the \ does

The backslash "escapes" the token following, preventing further interpretation.

It's an important mechanism in defining aliases like the example in your post, where "root" is aliased to include both the account of that name AND other addresses. Without the \ a loop would be created when the program interpreting the alias tried to dereference "root", which includes as a member "root" (which includes as a member "root", and so on..)

I believe that \root prevents that id from being passed through aliases again and (instead is) delivered to the LOCAL root account.

\root will save the message to (for example) /var/mail/root and forward it to [email protected]. Without it, only [email protected] will receive the message. The \user, other@address notation is very common for users that use ~/.forward files.