What is the main purpose of the `update()` method in MongoDB?

The update() method in MongoDB is primarily designed to modify existing documents within a collection based on specified criteria. When this method is invoked, it allows users to identify the documents that meet certain conditions and then apply changes to those documents. This could involve updating specific fields' values, adding new fields, or even removing fields, all depending on the update operators used in the command.

The nature of this operation is centered around altering data rather than creating or deleting records. This is why the correct choice focuses on modifying existing documents rather than introducing new ones or removing them completely. The other options, while they describe actions that can be performed in MongoDB, do not accurately capture the functionality of the update() method specifically. For example, adding new fields can be a result of using the update method, but the main intent is to change what already exists rather than to solely create or delete.