Back before municipal water treatment, all water sources were dubious at best. Drinking water from most sources was akin to taking your life into your own hands. Beer, as you know is boiled, therefore making the liquid safe to drink. Back in the day everyone drank table beer (named so because it was usually consumed with meals). These beers were low in alcohol (below 3%) and rich in nutrients (residual carbohydrates and yeast from lack of filtering).