Why is B the correct answer?

Hi, let's try to dissect this argument. The author starts by telling us that many theories aren't actually good, but then claims this isn't really relevant. Instead what matters is that they help people. In this case the conclusion is that it doesn't matter the theories are poor, which is supported by the fact that they help people. We are asked what role the final part plays. The final part does trump the concerns about the scientific validity of those theories. Sure they may not be valid, but validity isn't important. What's important is helping people. This, the fact that they help people overrides the concern about their validity.