It's important to have a backup plan. Sure, it's nice when everything goes swimmingly, but the chaos monkey will often get in your way. Your backup plan is an investment you make to increase your chance of success.
A side-effect of having a good backup plan, is that it forces you to think more deeply about your plans, or at least for a longer time. Think of it like a speed bump that slows you down and gives you more time to look around for hazards; you have more chance of avoiding incident entirely.
Some setbacks can't be dealt with on-the-spot. But are trivial to handle with just a little bit of planning. For example: what do you do when you're taking an exam, and your pen runs out of ink? With a bit of planning, and bringing (at least) two pens, you would've been fine.