you take into account Hofstadter's Law.
Convenience is a big one. Shelf life is another.
The water in the bottle is *not* the same as that from a garden hose or tap, even if the initial source was the tap, it is significantly cleaner biologically. Remember that tap water going into the bottling plant is filtered, chlorinated, dechlorinated and reverse osmosis filtered, all the while passing through piping systems that are sanitized with chlorinated water several times per shift along with the bottling line and then it's filled into bottles that have been sanitized as well. It's the same water and process as the soda bottled on the same lines.
Pete C.