A demo app should be considered a critical piece of the Ollie-UI starter kit.
The purpose of a demo app conveys expectations around suggested directory structure and file organization. It helps encourage consistency by codifying patterns for how developers should work with the libraries and the frameworks selected.
It shows example tests that are passing, so developers have a point of reference for various testing scenarios, naming conventions, file placement, and mocking strategies.
It provides a realistic example of a mock API, working in your domain. This gives developers a big head start, since referencing examples is often quicker than pulling up docs for desperate packages.
It provides a working, automated deployment, so there's already a clear recipe for use, for future apps. And it gives you a single place to codify your decisions. The demo app reflects teams' coding standards.
It's a place to update as you learn new techniques and patterns that you want to share with the team.
Finally, and perhaps most importantly, it offers an interactive example of the starter kit working in a realistic scenario IN PRODUCTION. This helps new team members understand what life is like working in the selected stack.