Just-in-Time Management Principal is used by pretty much all governments and businesses as a way to reduce storage costs. What it dictates is that goods and services are delivered on an as-needed basis. When I buy a can of soup from the local grocery store, it goes into the system, and on Thursday when the truck shows up, it brings a can of soup along with everything else that everybody else bought.
If that system ever fails, we will be desperately out of supplies almost instantly. That means no food on the grocery store shelves, that means potentially no water even when you turn your faucet on in your house. It's very important to prepare ahead of time for any disaster, zombie or otherwise, because without the regular infusion of deliveries that Just-in-Time Management Principal requires, there's going to be no supplies for you to go pick up after the [beep] hits the fan.
By way of example, in 2005, there was a trucker strike in Italy that didn't even make the front page of the local papers. It started on Monday, by Wednesday almost the entire country was completely out of gas and food. People drove to gas station to fill up their tanks to go to work, and were told, "Sorry. There's no more gas. The truck didn't show up to deliver it." That's what's going to happen anytime the system breaks down, no gas, no food on the shelves, no water to buy. Preparation is key, because without it, the Just-in-Time Management Principal dictates that we're all going to die.