What is the main purpose of using the _id field in MongoDB?

The primary purpose of using the _id field in MongoDB is to ensure the uniqueness of documents within a collection. Every document stored in a MongoDB collection must have this _id field, which acts as a unique identifier for each document. By default, if an _id field is not explicitly provided when a document is created, MongoDB automatically generates a unique ObjectId for this field.

This uniqueness is critical for various database operations, like updates and deletions, where you need to target a specific document. If the _id field were not unique, it would lead to ambiguities in identifying and managing documents. Thus, it serves as a fundamental aspect of data integrity within MongoDB. This uniqueness allows for efficient data retrieval and manipulation, which are vital for any database application.

The other options, such as linking documents together, categorizing them, or storing metadata, are not the primary function of the _id field. While links can be established through references and categorization can be done via other fields, those do not override the fundamental purpose of the _id field in ensuring that each document in a collection is distinct.