Caveat: I'm not medically or pharmaceutically qualified
> What could be benefited by not approving it with all the pertinent disclaimers about effectiveness?
It prevents the drug being prescribed (at great cost!) to people who are desperate to try anything.
What would be the benefit of approving such a drug? Who is missing out by not being able to take a drug not known to be effective? I would consider it malpractice to be prescribed something not demonstrated to have a measurable positive effect. The human body is vastly complicated - we shouldn't mess with it without good reason.
> And to turn it around, what harm would it cause society if it’s not approved due to inconclusive evidence and helps only some people? (Assuming no hard side effects)
If it's not approved, then I would argue no harm comes to society. What harm is done if a drug that wasn't available before, still is not? Nothing has changed.
> Keep in mind, there are already ineffective treatments that sell hope and don’t help.
Absolutely, and they are, in my opinion, a work of evil. But the crucial difference is that such treatments (I'm assuming you're referring to quackery such as homeopathy) contain no active ingredient. That's why they aren't regulated. They can't cause medical harm because they don't do anything.