Writing a unit test requires controlling all the input data. In case a method receives its input from other objects, there arises a need to limit the depth of testing so that each layer can be tested in isolation as a unit. This is when the need for mocking the lower level comes into focus.
Mocking can be done not only vertically, but also horizontally at the same level. If a method is big and complicated, you might consider breaking it into several smaller methods so you can test only one of them while mocking the others. This is another advantage of unit testing code along with its development; it is easier to redesign code for better testability at the earlier stages of its development.
