Because you get the convenience of having a document store with a schema defined outside of the DB if you want it, along with the strong guarantees and semantics of SQL.

For example: let's say you had a CRM. You want to use foreign keys, transactions, all the classic SQL stuff to manage who can edit a post, when it was made, and other important metadata. But the hierarchical stuff representing the actual post is stored in JSON and interpreted by the backend.