Mocks (or I guess fakes, really) can be useful for simulating certain scenarios that are difficult to produce in real life but your code needs to handle. For example if in an integration test you want to see how your application deals with high latency or errors in a service dependency, you can write a mock of that service and have it introduce arbitrary delays or errors.

Of course it helps to test against the real service as well, since it's often prohibitively time-consuming or impossible to model ever aspect of it, but I wouldn't discount mocks so easily.