The idea is by making it beer you keep safe water safe for longer. It doesn't sound particularly weird to me (I don't know if it was really a common practice in medieval Europe.)

Before hopping and such (didn't become widespread until the 1200-1400s) beer didn't really last that long. Without refrigeration, modern bottling and almost sterile conditions in modern factories beer will go bad very fast.

That's a hypothesis. Provide any data whatsoever that this was a consideration.

Beer also makes water a more pleasant color. That too could be a reason to drink it. Or, either idea could be irrelevant.