MongoDB is the fastest and easiest to scale schema-on-read document store.
So if your domain model is document orientated e.g. a star schema with dozens of joins, where you don't know the schema upfront or you have polymorphic relationships it is a really useful way to store your data.
That's actually kind of the point – if you're working on a new project whose requirements might change rapidly Mongo can be a really great fit (eg a toy project; a prototype for a new internal service; a hackathon; a pre-traction startup).
I would also add that the replica set concept that is based on Raft [0] allows for built-in high availability, so the individual servers can be maintained while the whole set is running and servicing clients.
