Sometimes it is just a lot easier and more query efficient to store a bunch of related data as just a hash or even with embedded hashes. Like, if you want to store a list of key/value pairs alongside a bunch of other data. Yes, you could do that via a bunch of tables and relations and joins and things, but conceptually if all that data can be seen as one self contained record, why spread it across a bunch of separate tables?

Sometimes a document collection is just a whole lot easier to reason about.