More than that, it forces iPhone-only devs to get with the program and make their apps usable on larger screens too.

I wouldn’t be surprised at all if next year they dissolve the iPhone/iPad distinction on the App Store altogether and maybe even remove the Catalyst toggle on the Mac App Store. If you make an iOS app, it’s also a full fledged iPadOS, macOS, and visionOS app too.

I certainly wouldn’t mind. On my Mac there are some needlessly heavy electron apps I’d swap out for their iOS counterparts in a heartbeat if that were possible, as well as some games that would run fine on macOS but their devs don’t tick the checkbox for unclear reasons.

I'd guess that they're meant to bridge the gap between iOS and iPadOS, if anything.