Even today, there are beliefs that it prevents STD transfers. Perhaps if jews are too cheap to wash themselves, there maybe some truth.

I think its more like why did ancients sacrifice animals. A mix of luck and how some retards benefit from placebo. I think religion overall flourished back then because all else equal, i'd bet on an army of retarded theists with courage over an army of atheists who (perhaps rightfully) fear death. Also, mating/natural selection is not always about the best. Why do we have 5 fingers instead of 4 or 6.