Beer is composed mostly of water. Often overlooked, the water used in a beer has a great deal to do with how it ends up tasting. Identical recipes brewed in places where the mineral content of the water is very different will taste different from each other. If beer has terroir, a place, it's based on the water in that place more than anything else. Large brewers that brew in multiple locations (e.g. Anheuser-Busch) need to constantly analyze and adjust the chemical composition of the water they use for brewing in order to keep the taste consistent.