thefutilist
Adventurer
One of the things that I like about sharing clocks openly is that it makes it seem like the characters are living people with lives in the city and they hear about things or get hunches about things and so on. I find clocks to be a great way to kind of support the lived in feeling.
I wouldn't put it this way, I'd use the far colder sounding 'it gives situation legibility', but they amount to very similar things. So why don't I like the idea? I'm still trying to work through it.
A GM certainly could use secret clocks to keep track of how some offscreen process is proceeding, but they could also just declare things happen at any time, because reasons (which may include in response to some action by the PCs).
They could declare things can happen at any time but there's fairly good reasons as to why they shouldn't and secret binding decisions (such as off-screen clocks) are one technique to stop them from from having to do that.