Besides, evolutionarily speaking, everything is about reproduction and raising children to sexual maturity. If women tend to prefer to mate with men who wash themselves regularly (there's strong evidence that's the case), and/or if washing regularly increases the chances of boys reaching sexual maturity (there's also evidence that's the case) then that's the behavior evolutionary pressure is encouraging.
Besides, evolutionarily speaking, everything is about reproduction and raising children to sexual maturity. If women tend to prefer to mate with men who wash themselves regularly (there's strong evidence that's the case), and/or if washing regularly increases the chances of boys reaching sexual maturity (there's also evidence that's the case) then that's the behavior evolutionary pressure is encouraging.