Ubuntu has not "disabled" the root user in the sense that the user account can be used, but there is no usable password set, so one cannot log in as root the way one can log in as a regular user. This means that
could not be used, because it requires root password, which cannot be used. However, if one is already working with root privileges, then "su" does not ask for the password, and thus using "sudo" to run "su" will ask for the password of the user
, and if it is correct (and the user is allowed to run that command with sudo), then "su" is run with root privileges, effectively switching the user to root (the dash in the end of the command affecting the environment of the newly switched user).
If there is only one user on the system, using "sudo" is mostly the same as if one just had a regular root user account, with the exception that
1) there is no need to explicitly switch to the root account to do things with root privileges and
2) the password given to "sudo" may be remembered for a short while, so several commands (within a certain time interval) may be run using "sudo" without having to type the password again each time
With multiple users on the system rights to "sudo" can either be given for one user only, making that user very much resemble a root user, or then the rights can be given to multiple users. With multiple users being able to use "sudo" the system may have several "admin users", and actions made may be supervised a little better, because they're done under a different user id, instead of several people all using the root account which means it may be impossible to say who did what (because they all do it as root). There's also a possiblity to restrict actions; real root user is not limited in any way, but users who use "sudo" can be controlled via /etc/sudoers by explicitly defining what they can run with "sudo". For example an admin user can be allowed to run certain commands, but only those, so s/he may not run "su", "sudo", shell or other things that would grant her/him access to the true root user account.
Using "sudo" is one kind of a two-edged sword: on the other hand it may ease writing directions for inexperienced users that require root privileges, it may ease system administration by allowing more than one separate administrative user account and it allows for restricting the elevated privileges to certain commands only etc., but on the other hand it opens one new possibility to exploit the higher privileges (causing more brainwork when thinking of giving "sudo" permissions), deviates the way from the old standards as mentioned by pixellany and possibly makes the whole privileges system more difficult to understand.