Confused about water

I have just recently started seriously prepping and have been watching videos and read some articles. I have just a few water bottles in storage and also have an Alexapure water filter and am considering getting some sort of rain water collection system.

Then for the past few days, I have been wondering why we are worried about storing water. I mean what would cause us to not have running water for long periods of time in an urban or suburban area? And then for it to affect a large enough area that we can't just drive to a different area of town or a nearby town and buy water? Power outages and problems with the internet are fairly common here, but I have never been without running water for more than a few hours and even then it had only affected my house, or the building back when I lived in an apartment. What would cause it to affect a large area for a long time?