In general, mocking is used in two different situations: when developing the unit under test, and when developing units that interact with the unit under test. In the first case, mocking helps to isolate the behavior of the unit under test from its dependencies. For example, consider a class that retrieves data from a database. To test this class, we would need to set up a database and insert some data. This can be time-consuming and expensive. By mocking the database class, we can stub out its behavior so that it returns predefined data. This allows us to focus on testing the logic of the class under test, without having to worry about its dependencies. In the second case, mocking helps to simulate the behavior of units that interact with the unit under test. For example, consider a class that sends an email. To test this class, we would need to set up an email server and define a rule to route incoming emails to a specific mailbox. This can be difficult to do in a Development or QA environment. By mocking the email class, we can stub out its behavior so that it writes emails to a file instead of sending them over the network. This allows us to focus on testing the logic of the class under test, without worrying about its dependencies.