> why I'd want to map my MacOS desktop onto the background of my iOS tablet

That's not what Sidecar is. Where did you get that impression?

Sidecar is just a means to use an iPad as a secondary display for a Mac — it's an app that takes up the whole screen. While the iPad app is running (and it can be dismissed both via the Mac and the iPad itself and resumed via the Dock), it's just an external display in the same manner as plugging in a monitor.

Very useful for people on the road who could do with the extra screen real estate on their MacBooks, people who use apps with a lot of floating panels that they'd like to get off the screen, or perhaps someone using Sidecar's Touch Bar panel in developing a Touch Bar interface on an iMac.

As a bonus, one can also use it as a sort of Wacom Cintiq substitute if they use an Apple Pencil, which kicks macOS' native drawing tablet functionalities (usually used for Wacom tablets and the like) into action.

Many such third party apps have existed for iPad (and iPhone) for a long time, such as Duet Display, Splashtop XDisplay — perfect for when you need a second display on the go.

Sidecar provides much better performance and simpler setup than any third party offerings — but only works with macOS Catalina, whereas things like Duet also work with Windows.

Yeah, just because you can do something, it doesn't mean you should invest heavily into it. Weird setup avoiding the fact that MacBooks still don't have a touchscreen. My point was that Apple now is trying to turn tablets into something useful on the desktop, i.e. like a Windows 10 device, which is dual mode.