What are the reasons for using an instrument approach on a clear day / CAVOK?
There are actually quite a few reasons to fly an instrument approach, especially one with vertical guidance (like an ILS), even if the weather doesn't require it:


It serves as a backup to the visual approach.  There are various visual illusions that can cause a pilot to fly an approach too high or too low and monitoring the vertical guidance can help to mitigate those.
It helps to ensure that you land on the correct runway, and even at the correct airport.  (Unlike this airplane.)
ATC uses it when pilots do not have the airport or runway in sight.  This happens quite frequently at large airports where they have to line many aircraft up on final.   The last guy is a long distance from the airport so may not be able to see it but can fly the approach.
It can be useful if it is hazy or the sun is making it hard for the pilots to find the airport.
Some parallel runways are approved for simultaneous approaches (as is the case at Narita for runways 34L &amp; 34R) so ATC will clear two airplanes for the respective ILS's.  This keeps them tracking the centerline of the approach instead of maneuvering visually where they might drift over into the way of the other airplane.  
Flying the approach on the autopilot leaves more time for the pilot to focus on other things and possibly catch something that might have been missed while hand flying a visual approach.
Sometimes instrument approaches are used for noise abatement reasons.


If I had to guess about the particular approach that you mention, I would say that it is probably company SOP (Standard Operating Procedures) to fly the ILS whenever possible for the safety benefits, and that Narita allows simultaneous parallel landings to the two runways so ATC would require it as well.