Apple, on their developer site:

The macOS version of your app can now be included in a universal purchase, allowing customers to enjoy your app and in‑app purchases across iOS, iPadOS, macOS, watchOS, and tvOS by purchasing only once.

John Voorhees, MacStories:

Prior to universal purchase, Mac apps were treated as separate products by Apple’s stores, which meant developers had to either charge separately for apps and, in some cases, jump through complex receipt-checking hoops to bundle their apps. This change should make the process of charging a single price or signing up for one subscription for apps across the Mac, iOS, iPadOS, watchOS, and tvOS much simpler and will enable cross-platform In-App purchases too.

Universal purchase will certainly make life simpler, both for developers and for users.