In the Middle Ages, everyone drank beer, because water was not safe to drink. The people then did not know that one part of the brewing process was actually what made the drink safe: boiling. Therefore, with modern water purification processes, we don't need to drink beer.
That's all well and good, but it ignores a couple facts that counter those conclusions:
(1) Distilled water will leach minerals from your body. So water boiled to extreme is actually bad for you. But then again, even if they did know about boiling to kill germs in the Middle Ages, they would probably not be distilling it.
(2) Water storage. Water will turn, get infected by germs, in storage. That's why sailors made grog with bad water and rum on ocean voyages.
(3) Wine was never boiled and was also a safe substitute for water.
(4) Fermented products have additional benefits from amino acids acids, trace minerals, active live cultures (if not pasteurized), etc.