You've just built a calculator, and now you want to test it. Well, you have to push buttons on a calculator, so you build a robotic hand. And that hand needs a power source and some intelligence. And you need to photograph and OCR the result from the calculator screen.

This is kinda how we build software right? A little bit of "our logic" (calculation), represented as objects/actors/modules which "do things", but intermingled with million-LoC dependencies like databases and web servers.

After a while it gets frustrating setting up the robot hand and OCR equipment for each test case. Maybe it's just easier to test manually, or skip testing entirely.

At this point you can have an epiphany, and realise you only care about the numbers going in and out of the calculator, not the button pushes and pixels.

Mockito swoops in and prevents you from having that epiphany, by making it easier to keep doing things the stupid way.

Instead isolating the calculation from any IO, you can now write things like: when(finger.pushbutton(1)).then(calculator.setState(1)) when(calculator.setAnswer(3)).then(camera.setOcr(3))

(I've mostly worked in Java, but it seems like other languages typically don't let you intercept calls and responses this way)