I have no clue either. I don't even like it as much on the iPad, although it makes more sense there. On the Mac, it's just a gimmick for me. Obviously, many of the codebase is shared between iOS/iPadOS and macOS. What Apple does by porting such features on all platforms is smart - they develop it once and make it available to the other platforms, claiming that it is a new feature for all of the platforms. It looks like as if they have added something new, but, at the end, they have really done the bare minimum. To me, macOS Ventura and iOS/iPadOS 16 are as mediocre updates as macOS Monterey and iOS/iPadOS 15 were. I hope stability and performance will be better, although, obviously, this is something we won't know for the next few months.
I will be looking on the forums and YouTube to see how people will utilise the Stage Manager both on the iPad and the Mac.