Well suppose you want to replace your old car, but you aren't sure if it'll save or cost you money. You do a few calculations and think about it -- but just how certain are you of your calculations? Maybe your data and estimates are off? Are you 60% sure that you won't have to replace <part> for another five years? Now on your graph, 60% on the x corresponds to about 42% on the y. So if you think you're 60% sure, the chance is probably nearer to 42%. And obviously with that information you can make a better decision.
You make choices all day, every day. It's tremendously valuable to know how accurately you can judge a situation. Are you overconfident, underconfident? Wouldn't you just love to know!?