I think it's very tough to describe the "use case" for RethinkDB (I used it in production two years and have expressed that I don't think RethinkDB did exceedingly well at any particular use case to warrant being used over other solutions). It might be easier to list the differentiators in roughly the order they were introduced:

JSON document storage

ReQL Query Language

JOINs

Easy Deployment

Administrative UI for monitoring, sharding, querying data

Change feeds

High Availability