Civil unrest, natural disasters, economic collapse, and aliens (not necessarily the interstellar kind). All of which could disrupt the power grid and/or water and food supply supply.
Retirement (financially) and natural disasters, in our case major storms like a hurricane.
During hurricane Ike our house (my now-wife's house, but before we met) was without power for two weeks. We have enough food and water stored to last us at least that long now.
Yes, yes, I know, some preppers look upon prepping almost as a religion and if you disagree with their prepping ways, you're labeled a heretic.
The over the top preppers can't prep for everything, but they're certain they can.
You say brain surgery and a heart valve replacement is needed, plus you're out of insulin, and wow, you just sawed off your hand, well partner, you should gone to medical school. What? You are a brain surgeon, but you don't have all the tools necessary to perform surgery?
I'm alone now and too far gone to worry about such things anymore other than to feel really bad for future generations. When I was doing some prep it was for economic collapse and the inevitable ensuing civil unrest.