Keeping tests fast and stable is critically important. This is hard when servers depend on many backends. Developers must choose between long and flaky tests, or writing and maintaining fake implementations. Instead, tests can be run using recorded traffic from these backends. This provides the best of both worlds, allowing developers to test quickly against real backends.
Video producer: https://developers.google.com/google-test-automation-conference/
Further reading: How the Google+ Team Tests Mobile Apps