The OP_MSG specification requires that global arguments like $db and $readPreference are appended to the command document. So as not to require the core driver to modify command documents that it doesn't own, or clone those documents into a new BsonDocument, create a new BsonWriter implementation that can append a list of BSON elements to the end of the top-level document. This BsonWriter will be used by the OP_MSG serializer to efficiently encode the command document to the wire.