It’s sometimes not enough to prepare for one thing going wrong. When things go badly wrong, it’s usually because more than just one thing goes wrong. Being low on gas is no problem, because there are gas stations everywhere. Leaving the house a few minutes later than usual usually isn’t a big deal either. A little rain might slow traffic down but not too much, so that’s not a big deal either.
But what happens when you’re leaving your house a few minutes later than usual and then realize that you also need to get gas before you drive to the airport? And then it starts to rain and there’s more traffic than usual. And an unusually long line at airport security. The combination of problems can result in a bigger problem, like you missing your flight. Better to fill the car with gas even if you don’t think you’ll have to rush out of the house tomorrow morning. Emergencies don’t give fair warning.