The immediate feedback provided by unit tests, which run significantly faster than the other two, helps me as a developer. Using a TDD approach, and frequently invoking my unit tests, the design is refined and failing code is identified quickly. At my current employer we've tied the unit tests into the continuous integration process so that if code coverage drops by 1% or more the build fails. That's a nice defense against having the code atrophy. I also like the closing statement from Martin Fowler's post, Test Pyramid:
In particular I always argue that high-level tests are there as a second line of test defense. If you get a failure in a high level test, not just do you have a bug in your functional code, you also have a missing unit test. Thus whenever you fix a failing end-to-end test, you should be adding unit tests too.
Integration and E2E tests have value, and I'm not arguing they be skipped, but I've found the greatest ROI comes with unit tests.
