The hostname can be specified in the
sudoers file as a specific host or a set of hosts. This allows an administrator to distribute a single copy of the file to multiple hosts without needing to tailor each file for each host. A user on one host may have
sudo access to a particular command only, while the same user has
sudo access to another command or set of commands on another host. The
sudoers files on both host could be left identical.
You should not delete the line that gives root the ability to use
sudo. Allowing root to use
sudo means allowing somebody that is already logged in as root to execute a command as another user easily (and also have it logged, for auditing).
A line saying
username=ALL(ALL) ALL would contain a syntax error. Using
username ALL=(ALL) ALL would give the user
username the ability to execute
sudo to run any command.
sudo group may not exist on your system. There is nothing stopping you from creating it though, and to add users that should have full
sudo access to that group. This facility allows you to administrate
sudo access by adding and removing users to the named group, so no editing of the
sudoers file is needed to add or to remove the
sudo access for a user.
wheel group is mostly used on BSD systems, and is traditionally the primary group for the root user on those systems. Again, your system may not have this group.