Can you imagine what kind of new things will now be possible? One word: design automation (okay, two words!). You want Artboards that showcase a font and its variations, like a Google Fonts page? There’s probably going to be a script to generate that file. There will be websites from which you can download freshly brewed Sketch files based on what you ask — say an image gallery, or a landing page, or a signup form. You’ll be able to pick your brand colors, choose a theme, randomize it, and voila! you have a Sketch design to start playing with. Someone could even build a Sketch equivalent that runs on the browser. The possibilities are many!
Additionally, aside from automation and cross-platform interoperability, switching to a JSON-based file format should make file versioning — tracking changes to a document over time — much simpler. Currently, if you have a Sketch file in version control, any changes you make are tracked as a single, opaque modification. With the new file format, you should be able to see not only that the document changed, but also what parts of the design were modified.
Using a JSON-based file format will let designers leverage the power and collaboration of version control, without sacrificing the context of their changes.