Without a crystal clear understanding of the processes when a team works on a software product, it can be tempting to think that all the problems stem from under-qualified QA engineers who click around randomly and ruin the hard work of the whole team. However, the value and purpose of the quality assurance process are not transparent without documentation. That's where a test plan and test strategy can help.
Even for those who are well aware of the processes, there's still the problem of measuring the quality QAs provide. If you don't measure quality, you don't have any control over the testing process or any ability to anticipate the results. So how should you know what exactly you paid for?