The water in most municipal systems is chlorinated in some way, and kept under pressure while in the system. Once the water is no longer under pressure, the chlorination compound can be degraded by UV light (sunlight), heat, gaseous diffusion, and time, to the point where it is no longer in the correct form and concentration to be effective against biological contaminants.
So it isn't as though it "goes bad" like food does, but the safety factor against unsafe handling goes away.
The other problem is that materials from an unsuitable storage container could slowly leach into the water, like plasticizers or metals. If you store water in mason jars and seal them as though it were food to be preserved, that water will not "go bad" until the lid rusts through, and depending on the possible contaminants, you can probably still drink it after bringing it back up to a boil.
As for myself, I keep much of my water stockpile frozen in plastic 2L bottles, because it keeps the freezer temperature more stable under normal circumstances, and keeps the rest of the stuff in the freezer frozen longer in case of power outages. You can always pull one out and drop it into a cooler for short outings, to keep the drinks and egg salad cold. And then, after it melts, you can still drink it. The only tradeoff is that it takes up space in there.
