A simple but sudden question almost puzzled me:
“Why should testers test, not analysts, developers, or users?” I will try to quickly substantiate, but, most likely, outside help will be required, such formulations require multilateral analysis and coverage, and, despite many years of possession of the topic, it may take time to think.
Facts, arguments, opinions
Let's start with general education: a tester is a role in an IT team. Not the most, let's say, high position in terms of rank and salary, but requiring special knowledge and skills.
1. If the leader does not understand the benefits of role division, then he and his team are not methodologically mature. Methodologies RUP, MSF allow analysts, but not developers, to combine the role of a tester. Extreme methods go further - almost all roles can be combined there.
2. If the leader tries to combine roles, then this is due to economy or due to a misunderstanding of psychology. Savings - most often from poverty or from greed.
Psychology
The psychotype of a person is important here:
1. Developers are creators.
2. Testers are disruptors. The effectiveness of testing, other things being equal, is higher for someone who deliberately breaks the system.