What is the usecase of MongoDB?

I am really wondering in which scenario it actually makes sense to use MongoDB. As far as I understand it right now, it basically has collections of json files (a bit more efficiently stored) which are completely unorganized. When actually using mongodb, most people use a lib like Mongoose which puts a layer on top of it that artificially creates schemas and relations.

So my question is: why would anybody wanna do that? When I need relations and a schema, I just use a sql database.

To me, using mongodb seems like a really poor design decision, wich has only drawbacks compared to a sql database. So why are so many projects using it?