Well even if you are sure about your intuition in the knowledge that this confidence by humans has led to huge problems and tragedies in the past, you still want to have a good estimate of the effect size.

How much resource to allocate to this treatment vs other treatments having a positive effect.

Somewhere there ends up being a trade off that I will caricature as "Do we want another hospital OR another park?"

(All the above is idealised, the biggest win that moves the efficient frontier outwards is effectively fighting corruption and being vigilant defending the gains made. This seems to be universal across countries and cultures).