Data without a schema is useless. You get a document from MongoDB, what do you do with it? Read some fields? You need to know the names, types and meanings of those fields. That’s a schema.
When people say that MongoDB “has no schema”, they really mean that it does not enforce schema the way SQL databases do. MongoDB pushes schema concerns up to your application level, where you can handle them more flexibly. For example, in order to add a new field to your documents, you don’t need to do an all-or-nothing ALTER on your collection—potentially millions of entries. You just add that field to your ODM (Mongoose) schema and you’re done.