They are on a path towards merging iOS and macOS and fully merging their iPad and MacBook product lines. They’ve been on this path for a while, you can see it in things like moving the MacBook to ARM, adding mouse support to the iPad, making the iPad OS more desktop-like and making the desktop OS more iPad-like.

This is just the next step on that path. The eventual goal will be all of their devices will run the same OS with only subtle variations.