[mbrock]

The classic justification is that mocking encourages modular code where each unit has shallow dependencies with well-defined interfaces.

This also means that a mistaken change to one important unit will not break the entire test suite. Sure, the entire program will break, but it's nice to get a single failing test.

Mocking also gives a very straightforward way to simulate interactions with collaborators. You just say "given that the HTTP module returned a 404, this request should be sent to the error log," instead of initializing those two modules and arranging for the desired case to happen.

There's a very old discussion about decreasing coupling and increasing cohesion that's super important to the whole motivation behind TDD and that nobody seems to be very interested in anymore...