“So why not just save some water from the faucet BEFORE the storm comes?”
Another good question.
Because tap water doesn’t keep as well as bottled water except when refrigerated. If the power goes out it can get foulded with bacteria within a few days.
This can happen with refilled 5 gal.s too. Before I fill my 5 gal. jugs I put a little bleach and water in, put on the cap and shake. Then it’s ready for refill. This can be done with home containers and the faucet but it’s a bit much and you don’t know how contaminated the container and water is to begin with.
I keep a packet of pool shock handy - mix with water to make a bleach for water purification.
You are way, WAY too paranoid about bacteria in tap water, in normal circumstances, unless maybe you are talking about tap water from a suspect / untreated private well, or have a compromised immune system.
In normal circumstances, municipal tap water in a sealed disinfected container* will keep @ room temp. for many weeks if not months, because it has lot of chlorine and (more commonly, these days) chloramine in it. Chloramine is actually hard to remove, because it doesn’t evaporate out easily, and must be boiled out over a considerably longer time than chlorine.
https://aquanswers.com/how-long-does-it-take-for-chlorine-to-evaporate-from-tap-water-entirely/
*Wash it out well, then fill it with cold municipal tap water; if paranoid let it set for a day, empty & refill with same.
“Store bought” bottled water should keep even longer, but if your municipal tap water properly stored does not keep for at least a month, something is seriously wrong with either the source or your storage method. This is not Mexico - I hope!
I suppose if one has drunk only bottled water from birth, then one’s resistance to likely remnant bacteria in virtually any water source might be low...
“Because tap water doesn’t keep as well as bottled water except when refrigerated. If the power goes out it can get foulded with bacteria within a few days.”
Tap water from a public water system contains Chlorine. It will keep for ages if you store it in a clean sealed container. You can also add Chlorine if the water does not already have it. Just buy regular Clorox bleach and find a chart on line for the amount to add.