It was in common use against diseases prior to antibiotics. Used to reduce the percentage of patients converting to pneumonia, when pneumonia was largely untreatable and highly lethal.
It was tested and shown to increase immunity cells, particularly the monocytes that are reduced by Covid-19 / Wuhan China Virus as part of its attack strategy.
It has the potential for a large decrease in ICU cases. No joke, this is an important video. It covers the theory and evidence.
Then, some fascinating history on it. In some ways more important as it covers ways to do it and historical evidence for efficacy: