I know it can be used for that, but I don't expect it to... I fully expect it to be a "let's just dump the iPad app over and be done with it" thing. Even Apple's own new apps are way too iPad-like.
For the same reasons that developers embrace Electron as much as they do. The apps it provides are way too resource hungry and a UI mess (not fitting into the platform at all) but it allows them to "fire and forget" on each platform. Way too often the Mac version is just a tick in a box rather than a well executed effort that actually integrates well into the platform. Again, Electron produced a few gems (Visual Studio Code as one of the examples) and a whole load of trash (Skype for Business, MS Teams from the same publisher, which definitely has the know-how AND the money to do things right)
The absence of such porting technologies forces developers to go back to the drawing board and in the process to determine how best to make an app fit inside each platform. This has over the years generated many exceptional Mac apps. While we all balk at the crappy generic Java ports (enterprise software in particular) that have been released for the platform over the years.
The point is: If you're really committed to doing things right, you will usually go for the full SDK option rather than what's basically a porting layer. iOS and macOS can already have a lot of code in common so two completely separate codebases haven't been necessary since the inception of iOS. For the UI layer yes, but those are different for a reason.
I'm sure this will drive Mac app abundance but I'm sure it'll be very few gems and a lot of half-hearted iPad ports.
Anyway it's here now... Let's hope I'm wrong and you're right!