A highly trained tester should have knowledge to translate specifications into testable requirements (s.m.a.r.t), so they can construct test scripts. That tester and his team should be able to verify if the product is conforming to the agreements and expectations of the client. Tne goal is to certify that the client can perform the actions for which the application is being developed.
By making testable specifications it can be determined if something works or not. By having the test scripts finished before the project, the project manager can be informed about progress, because there is a list of working and nonworking function points or nonfunctional requirements.
Testing should not be aimed at finding errors, but at defining product quality.