An app usually sit between the database and the user, isolating them from each other. This demo does not isolate the database, it is just a proof of concept.
Changefeeds are for getting updates to the results of queries on the database. So for example, getting the top 5 users ordered by their score. The app leaves a connection open to the database, and the database pushes changes to the query results whenever they happen.
The app can then also have a persistent websocket connection or whatever to the client, so you're able to get push architecture all the way from the data source to the client.