From an administrative side: being able to point explicitly to rules that are being broken to shut down an endless "why is X a big deal" and "but he's doing Y and he's not banned".

From a user side: being able to see where the lines are, and being able to see an explicit reason for a punishment. Being able to point out when others are breaking explicit rules. Being able to see and point out hypocrisy.

This does require admins to actually follow the CoCs and to act on them, which I think a lot of people in this thread are actually bitching about. Everyone who says "CoCs don't do anything by themselves" are right, but seatbelts also don't do anything unless they're plugged in, but we should still have 'em in cars.