I just pulled up a video that I remember having particularly bad comments, and the comments were gone. I guess that channel does have a code of conduct, even if it's not formalized. I don't know that publishing a CoC document "solves" anything, but at the very least it acknowledges that the sort of conduct I was illustrating does exist and should be discouraged.
Perhaps it's along the same lines as the "don't piss in my pool" sign. Some visitors may see it as a sign that you care about water quality and will feel safer jumping in. Others might wonder if it indicates that a lot of people do piss in your pool.
Arbitrary notions of safety and conduct. People know how they should conduct themselves, and those who will do something wrong don't care.
Arbitrary notions of power. Does enacting this over the members of this community improve anything? No. There are no issues within it to begin with. Nobody is rampantly "pissing in the pool" on a daily basis, hell even monthly basis at that.
Arbitrary notions of acknowledgment. Do you read through every ToS you've agreed to? No.
Perhaps it's along the same lines as the "don't piss in my pool" sign. Some visitors may see it as a sign that you care about water quality and will feel safer jumping in. Others might wonder if it indicates that a lot of people do piss in your pool.