Even kids drank (weak) beer, in part because the alcohol content kills pathogens. Even today clean water can be tough to find for some folks.

That's an old theory, but the whole "water was unsafe to drink" thing has been debunked pretty well.

The bigger reason is that beer provided an excellent way to consume a lot of calories fast, in a moderately shelf-stable (~3 days) format. You could carry your meal in a water container, which you needed to take anyway.