In places known for reliable water systems, the unexpected can happen. Even if the water commission swears the tap is safe to drink, you might not want to if it smells or tastes bad. Stocking up on water turns these situations into non-issues.
Have you ever doubted yourself because someone has told you you’re prepping wrong? You’re going to waste money, time, and effort. So you go study all the different ways and methods of safe prepping, get overwhelmed, and end up doing nothing.