A good QA person is basically a personification of all the edge cases of your actual production users. Our good QA person knew how human users used our app better than the dev or product team. It was generally a competition between QA & L2 support as to who actually understood the app best.

The problem with devs testing their own & other devs code is that we test what we expect to work in the way we expect the user to use it. This completely misses all sorts of implementation error and edge cases.

Of course the dev tests the happy path they coded.. that's what they thought users would do, and what they thought users wanted! Doesn't mean devs were right, and frequently they are not..