visudo checks the file syntax before actually overwriting the
If you use a plain editor, mess up the syntax, and save...
sudo will (probably) stop working, and, since
/etc/sudoers is only modifiable by
root, you're stuck (unless you have another way of gaining root).
Additionally it ensures that the edits will be one atomic operation. This locking is important if you need to ensure nobody else can mess up your carefully considered config changes. For editing other files as root besides
/etc/sudoers there is the
sudoedit command which also guard against such editing conflicts.