I have always done this too, but I think I do it because I grew up in the boonies in snow country so there was always the possiblity of being stuck for a while somewhere.
The other day we had to take a horse to the vet- the vet was backed up with emergencies and we had to wait for hours for him to wound check our horse. No vending machines- we left early and missed breakfast- nothing near by to buy anything- if we leave we will lose our place and the mare really needed to be checked. Thank goodness I keep snacks like peanuts, peanut butter crackers, trail mix and water in the truck with my Mary Poppins bag as my kids call it. It wasn’t even an emergency situation but we were stuck somewhere and were hungry and thirsty so it came in very handy.