Explain the Use of New BsonDocument

I am new to MongoDB and I am interested in developing with it in the .Net environment.

I have a question after completing the .Net Developers course.
During the course there was a lot of times when the New BsonDocument keyword was used… While I understand that the use of this keyword instantiates the BsonDocument class, I am confused as to the amount of times it is used. Consider the below example:

        private BsonElement BuildRuntimeBucketStage()
            return new BsonElement("runtime", new BsonArray
                new BsonDocument("$bucket",
                    new BsonDocument
                        {"groupBy", "$runtime"},
                            "boundaries", _runtimeBoundaries
                        {"default", "other"},
                            new BsonDocument("count",
                                new BsonDocument("$sum", 1))

Why are you calling the new BsonDocument 4 times? In my programming experience, (Which is limited) I was led to believe that so long as you can access the instance in which you instantiated the class, there was no need to re instantiate it.

What am I missing here?

I am trying to learn when I need to call a new document and there is VERY limited documentation on this. Is there a reference that I can read? If so, can someone provide a link so I can read it. If it is just the mongo driver, that doesn’t really explain to me why there is a need to call a new instance so many times.

Is this the same as the $AddFields: keyword?