To be clear, since the article doesn't mention it: this is the same progressive web app functionality you can use today with Chrome. I find it really useful, I'm excited Safari is getting the same functionality.
One difference is that Safari web apps can run independently from the browser, whereas those generated by Chrome and other Chrome-cousins have to launch the parent browser to run. As someone who uses multiple browsers Chrome’s behavior was always irritating.
Another is that they have entirely separate cookie, storage, etc pools so whatever you do in your main browser won’t impact the web apps and vice versa.
Safari/WebKit also tends to be a fair deal easier on battery life, which makes it more practical to keep these apps open in the background.
For my use cases, Chrome's implementation is worthless, and I have used tools like WebCatalog, Coherence X, Unity, and previously Epichrome, to achieve this.
The problem with these tools is they are usually 1-person operations and the maintenance is too hard (browser stuff moves fairly quickly) and they tend to get buggy and unwell after a couple years.
Having standalone web apps — with their own processes, application layers, and sandboxes to prevent cookies/sessions from leaking between them — built into the OS is the most exciting macOS feature in years, IMHO.
