> “your humours are imbalanced and you should be bled, you’ll feel better” is not far off from “your iron level is too high”, though…

The difference is that now we have both empirical evidence that it works, when it works, what the side effects are, and generally why it works. As opposed to cargo culting, poor cost/benefit analysis, and inaccurate understanding of what's actually happening.

>> those are for cleanly healing scars.

My go-to for that is Iodine.

Medical leeches are used to encourage blood flow, particularly after reconstructive surgeries; the benefit is not from the leech sucking your blood, but from the bleeding that continues after the leach is removed. (Their saliva has anticoagulant and anti-inflammatory properties.)