As I see it, a CoC gives good leadership a way to punish people who are assholes without causing too much of a backlash. Without it people would argue about fair process, not knowing it was wrong, cultural differences, etc, etc. It's hard to enforce a don't be an asshole rule when you mostly have soft power at your disposal. Of course, it also gives bad leadership a tool with which to punish people without causing too much backlash.