The app's transition from tablet to desktop (and MacBook) has been enabled by Catalyst, Apple's framework for making iOS software run on macOS. It's not entirely the same app as it was on the tablet, and now Swift Playgrounds works with the mouse, enabling click-and-drag for you to build your programs. And, with iCloud integration, games and projects that you start on the iPad can be finished with macOS, and vice versa.
As well as widening the number of people who can play the title, the move also shows Apple's commitment to making Catalyst work. As Computerworld points out, the move comes a couple of weeks after Apple allowed developers to begin building universal apps for both iOS and macOS machines. In March, users will be able to buy an app on either app store and use it on both platforms, hopefully juicing macOS development.
*And adults, don't write in.