This applies more so to good quality lager, some of the more well known lagers loose a lot of goodness because of the accelerated brewing through the use of chemicals. I believe in the days London's water was completely unsanitary and sewage drainage would regularly contaminate water sources it was common for people in certain areas to drink beer instead water, in fact some neighbourhoods close to breweries showed less plague related deaths, as the beer was more higienic due to the fermentation process. Beer loves you, and you should love it back.
I thought it was because the water was boiled as part of the brewing process & it was common accross medieval england to drink beer rather than water as a side note, on holiday in Goa some time back , all the guys were drinking kingfisher lager in the evenings the ladies (inc my ex) used to drink bottled water In the mornings the guys would be queuing up at the chemists to get anti sickness treatments for the ladies cheers (hic)