Yet another example:
Loan Formulas
I can work out how long it will take me to retire, how long it will take to pay off a loan, the effects of increasing my loan/retirement payments, etc.
I didn't know I'd need to know it. Never studied it in school. But, because I knew calculus, I could figure it out. You'd have to pay some smart-ass kid, and trust they knew what they where talking about, and trust your future to their knowledge...
Calculus is like literacy: the ability to read. Some people can't do it, some people can do it well enough to "get by", and others understand it enough to really use it. Being able to read fast/accurately won't help you get a job at the corner store, and you can probably limp on by as even an executive without high end literacy, but god damn it does it help.