In MongoDB, the _id field is automatically created and assigned a unique ObjectId value when a new document is inserted. This ObjectId is a 12-byte value that includes a timestamp, a machine identifier, and a process identifier. Because the ObjectId is globally unique, it ensures that each document in a collection has a unique identifier.
When _id is stored as a string instead of an ObjectId, the uniqueness of the identifier is not guaranteed, and there may be a risk of collisions or duplication. This can cause problems when trying to retrieve specific documents from the collection, as you may not get the expected results.
In addition, ObjectId values have some built-in functionality that can be useful, such as the ability to sort documents by creation time. When _id is stored as a string, this functionality is lost, and you may need to implement your own sorting mechanism.
However, there may be situations where storing _id as a string makes sense. For example, if you are integrating with an existing system that already uses strings for document identifiers, it may be easier to store _id as a string in order to maintain compatibility. In general, though, it is recommended to use ObjectId for the _id field in MongoDB, as it provides better functionality and guarantees uniqueness.