Why do we do this?
Let's let BMR777 explain that one: Adding an extra field onto the users table requires you to edit register.php so the SQL query for creating a new user matches with the new database table structure. Failure to do this will BREAK the user registration system.
That error occurs only if the system cannot automatically log in the new user after registration, which normally should never happen. What modifications have you made to register.php or to the script in general?