Exactly. Let's say a major company have a few applications which require MongoDB, and the rest of their applications are running on Postgres.
With FerretDB, they can migrate the app off MongoDB as you said, and keep Postgres only. Therefore they don't need to maintain internal knowledge on how to run MongoDB, or pay for MongoDB to run it for them (in which case they are not in control of their data, because it is all under MongoDB's account...).
With FerretDB, they can migrate the app off MongoDB as you said, and keep Postgres only. Therefore they don't need to maintain internal knowledge on how to run MongoDB, or pay for MongoDB to run it for them (in which case they are not in control of their data, because it is all under MongoDB's account...).
This is one real world user example.