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Except that leeching has benefits

It does! I wondered if someone would point that out. (Though blood letting does not). https://en.wikipedia.org/wiki/Bloodletting

Blood-letting also has benefits in some cases! For example, venesection is a standard treatment for haemochromatosis: http://haemochromatosis.org.uk/haemochromatosis/treatment/

But he referred to the notion of how it was widley used in old times and didn't do anything.