MongoDB/MySQL - auto increment vs object ID
In MySQL, or almost any other RDBMS, it is common to have an ID for the row (object), so you use it in other tables to make a link between the tables (Relations, or PK and FK if you prefer). This is part of the normalization process that is recommended for most types of DB design [there are exceptions though].

In MongoDB however, the general way or design is to denormalize. i.e. the document (row) is stored with all its related objects. 

To make it more clear:

In MySQL you would have two tables: cars and car_types. In cars you only store the ID of the type of this car.

In Mongo DB, you would have one collection called cars, and the document in this collection would have a sub-document that contains the type's info. (You still can have a separate collection for types, but you don't store only objectID in car's collection)

HTH