[david_draco]

Unit and Integration testing is great for decreasing cognitive load too. When you are staring at an error stack trace of a complex code base, and go through mentally what could have played out to cause this, it's great to have confidence in components due to testing. Hypothesis/QuickCheck is allows dropping entire classes of worries.

[supernuova]

Yes, exactly! If you can trust that each unit is working in all the ways covered in the tests, you can focus on the unit you are developing and not have to keep the other unit in your mind while working on it. And if there is an untested edge case you think might be the cause, you can test that edge case independently from the unit where the issue is occurring, and confirm if it produces the expected result.

Good, trusted unit tests are the difference between encapsulation reducing or increasing/complicating cognitive load. (And similar but between components for integration tests).

That being said, there will be rare times that the issue is due to something that is only an edge case due to an implementation detail several units deep, and so sometimes you do still need the full picture, but at least it lets you save doing that until you're stumped, which IMO is well worth it if the code is overall well-designed and tested.