the engineering team are usually great at writing tests that test their code, a good QA can test alongside them to find cases they've missed and issues that automated code tests can't find. The QA person doesn't have to spend time checking that the app basically works, they can be confident in that and spend their time testing for other 'qualities' But yes, I've known QA teams that will only find bugs that no one cares about or are never likely to happen - often because they are not trained on the product to be able to dig deep