As already stated, "2" is not used for any specific purpose. You can set up a new account with a user_id of 42093 and make them an admin, delete the original account and your board will still function.
As to why they don't use auto-increment, well, that feature of MySQL does not support negative numbers, and the -1 value reserved for the "anonymous" user prevents that. Why not use 0 or 1 as the anonymous user? Don't know.
0 is not a good idea because checking for zero (as opposed to false) requires special logic. So it was probably just easier to use a -1 value.