I don't think anyone said it was necessary, especially in modern times. But back in the day, it could contribute to reducing the multiplicity of infection of microbes in drinks. Thus, it was clearly worthwhile enough that some populations evolved the ability to metabolize it more efficiently. The other group probably evolved a better immune system instead, or alternative ways of cleaning their food and drink (e.g. the chinese drink a lot more hot water than cold, but that comes with its own costs/tradeoffs).