There are actually a lot of reasons. First, I'm a mom.
Having kids makes you look at EVERYTHING differently. I got pregnant with Izzy right after Katrina, so all of the horrific events that occured during and after the hurricane were still fresh in my mind. Plus we lived in Southern California, where everyone jokes/lives in perpetual back-of-the-mind fear of "the big one." Some of the more paranoid projected that, if "the big one" (earthquake, btw) hit the area I lived in, it would be far worse than Katrina. So, on a long walk with Izzy one day, I came to a few realizations. I was not just responsible for my own well being anymore- I was responsible for this little creature. If the worst were to happen, I would need to be able to care for and protect her. And in order to do that I would need to be prepared.
But I also don't believe in living in fear. And that is another reason to be prepared- living without fear doesn't mean ignoring the worst possible thing that could happen, it means preparing for it and knowing that you can survive it, then setting aside thoughts of worst case scenario. You DON'T have to think about it anymore.
Many Christians use their religion as a reason to NOT be prepared- if the worst happens, God will provide. Except that God didn't provide for the little girls who were raped in the Superdome. No, I don't blame God for that, my point is that God only provides to those who provide for themselves. Assuming that you don't have to prepare and God will take care of you is like being idle your entire life and assuming that God will pay your bills and buy your groceries. In Kitchen Stewardship's preparedness series last week many people make the point well that, as long as your mind and heart are in a prayerful place (and I interpret that as not a fearful place), stocking up and being prepared is consistent with being Christian. Those of you who know me well know that this was not a huge factor in my own determination to prepare- I believe in God, but I don't consciously think "does the bible/pope/etc say this is okay" with every thing that I do. I feel like spirituality is more subtle than that, and that if I feel moved to do something that isn't harmful to someone else, I often feel that it is God sending me a message, even if it doesn't jive with something that the pope or the bible says.... but if I get started talking about the why and how of my spirituality this is going to be a very long and rambling and off topic post, and I simply shared my perspective on this for those who may be using their religion as a reason not to be prepared.
And there are many less than catastrophic situations where a little food stockpile or some emergency supplies will help you immensely- if someone in your household looses their job or can't work, at least you know you'll be able to feed your family- on this topic, check out The Prudent Homemaker. If you find yourself out of power for an extended period of time in the winter and you have a generator or a wood stove and firewood stockpile, you won't have to leave your home (this is not that uncommon in the rural midwest- after an ice storm in January 2007 one of my parents' neighbors was out of power for a month... thankfully they have a generator, but can you imagine how much they spent on gas that month?).
I can tell you why I prepare, but I am no expert on the how. I know that I am buying things slowly so that it doesn't become a budgetary strain. I also looked up and printed out lots of survival info, especially ones that are frugal like solar ovens, sterilizing water using bleach or solar energy, home remedies. It bears remembering that in some emergencies you would either not have electricity or not have web access, so you will need to be able to have this information on hand.
A great place to start would be the culmination of Kitchen Stewardship's Preparedness week, The Real Food Preparedness Extravaganza.