This. There's a thriving open source ecosystem around iOS that is largely unavailable to macOS developers because of fairly arbitrary and unnecessary API differences in frameworks.

Unifying the API will open a lot of doors for macOS devs to use iOS libraries and provide a boost to macOS development.

I think those thinking about UI convergence are missing the point, and it's doubtful that this is what Apple has in mind. Having unified APIs makes dev life easier, and having official linkages between desktop and mobile apps makes it easier to do things like document sharing between mobile and desktop versions, or sharing of purchases (so you don't have to buy the desktop/mobile version twice).