Control center is responsible for drawing a bunch of the ui on the top right of the menu bar, things like the clock (many comments here seem to be saying this is related to showing seconds), Wi-Fi, sound settings, etc.
It also shows the annoying overlay bit when you accidentally open it :D
Indeed! When "Display the time with seconds" is enabled for the menu bar clock, Control Center sits at 0.8% CPU activity instead of 0.0% on my Mac.
I think part of the confusion is that there's the feature "Control Center" and the process controlcenter so you would reasonably think it's only involved in the marketing feature's UI.
Couldn't it render on demand (on click)?
Or when the mouse starts entering the target before the click at least?