TL;DR Each mock is an invisible line in your test that says "nothing past that line matters". It is up to you to figure out where and why to draw that line. For example, one must always draw that line for network requests in integration tests. Because something like an external server mustn't affect the results of your tests in any way. Here we are grasping at something tangible, at last. Mocking is your level that controls what affects the test results and what doesn't—because that, in turn, controls what the test is about.