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.

Changefeeds are meant to be used by an app that isolates the user from the database.