I have seen several different possible needs for operating (or at least providing input to) a Mac from an iPad, or other iOS device.
Running videos from your Mac, operating a control console, or using the iPad as a remote for several different types of activities, etc.
Often that is because the iPad is naturally a very portable device.
I expect the opposite would be possible, but can you offer a scenario where you would actually need to control an iPad from your Mac?
(I can't actually come up with one )
My scenario is, my Mac is at one end of the house, my iPad at the other, and I am not (yet) in the habit of carrying it around with me. Occasionally, but not often enough to make this issue terribly important, I want to run an iPad app for some info to use on the Mac without making the hike.
I do use iCloud for files, notes, contacts, calendars ... Let me elaborate ...
I handle finances from my Mac. Occasionally I have to prepare a paper deposit slip with dozens of check entries. The Calculator app does have a paper tape "feature", but it's useless for this purpose. I presents multiple items per line separated by + signs. Without making an exhaustive search, I have not yet found a Mac app to do what I need.
However, a quick search of the iPad App Store turned up exactly what I want for $4. I would like to open a window on my Mac showing the iPad, run the app there to prepare my tape, and print it (via AirPrint) to the printer beside my Mac.