IIRC it is not needed for MongoDB and that field is generally used by the bookshelf connector to determine what database is being used (since our bookshelf connector supports 4 different databases). It was likely added in case mongoose supported other databases besides just MongoDB.
If it’s working without it, then it’s not needed. To that end, keep in mind we will be removing support for MongoDB in our v4 and compatibility for MongoDB will be picked up by the MongoDB team on their own after the v4 release.