We are simply investing in things that we believe will go up in price.
I can buy a can of tinned fruit for 20p. At minimum wage of £6, I can work for 1 hour and buy 30 of these cans, meaning 1 can costs 2 minutes of work. If SHTF, a can of tinned fruit will be worth a huge amount of money.. maybe you will be able to buy 1 single can if you work for 6 hours straight, meaning 1 can costs 360 minutes work, meaning the "price" of the can has increased by 180x, which is a mega-return on your investment.
Obviously our priority is survival, survival, survival.. but I tend to think of it in this way, too. I believe that food is never going to be cheaper than it is now, meaning no matter how much you "invest" in it, you will never lose. The same goes for all other survivalist items.