This is mostly an attempt to make the unwritten principle a written one.
The principle, I think, is that we want to help site members improve their questions and answers, so if someone leaves an answer which deserves a downvote, it is helpful and constructive to leave a short comment educating the writer about the reason.
A comment can completely change the tone of a downvote. I am sometimes frustrated by downvotes I receive when I don't understand the reason. On the other hand, a helpful comment can make me think "fair enough - I was wrong there".
I can certainly see some situations where not commenting is perfectly OK:
- Others have already commented and made your point for you
- The question / answer is obviously spam
- There is some other clear indication of what is wrong (I can't think what this might be, however).