Faster tests

If you run hundreds of tests, you don't want to hit a real API all the time. This slows down your test suite quite a bit. The faster your tests are, the faster you can iterate over code changes.

Deterministic

Can you run integration tests on your notebook? Testing against live APIs require those APIs to be online all the time, and you have to be online as well. Free yourself from those dependencies.

Accurate

One reason for manual mocking is, to get the exact same response every time your app makes a request. Not coding all those mocks for API responses speeds up your development process.