Let us start by stating that not to meany people really think a zombie apocalypse is going to happen. The reason this is a common theme for preppers and survivalists is because it is the worst case scenario. It means help is never coming. You are not just trying to hold out for a couple of weeks until FEMA gets here. You are not safe walking around, and must be stealthy. People can be dangerous because they can be infected.
The reason most preppers and survivalists use this as a gauge to calculate their preparedness is because if you are preparing for being on your own (which is something no one will ever be truly ready for, except, perhaps, astronauts heading to Mars,) you are much further ahead for expected SHTF, like a blizzard knocking out your power and blocking you in. Now you have no heat, no access to outside food supplies, no additional water. Without heat, your pipes may freeze, and could destroy your house before to long.
We had a situation where a car knocked out our power in the dead of winter. We had no power for about 18 hours. No power, no heat, and 2 little kids sleeping in bed. After this, we noticed a hole in our plans. We are not ready for an apocalypse, but we are better prepper for a small SHTF. We now have a propane heater that uses the same fuel tanks as our propane camp stove. This provides redundancy and dual preps. We can cook and stay warm. Another thing about propane is that it is common. If need be, you can find it on peoples grills, stores and gas stations. We have gained a new prep now. Long term plan is to put a wood stove in the house as a supplement to what we already have.
Basically, when I mention the Zombie Apocalypse or something outlandish, this is the reason behind that. It is just a no one is coming to help no holds barred SHTF the like of which we have never seen. If you prep for something like that, something smaller will seem more manageable, especially since most things will be short term.