I always yearned to understand a practical reason to learn calculus.

I use my understanding of second and third derivatives and the risks and how they affect the likelihood of black swan events - to choose (strongly influence) who loses when playing a game of "Liars Dice". So there is that, I guess.

On a more serious note, lots of things in personal finance are a bit easier to understand with a functional understanding of derivatives and integrals. It's not critical, but it makes stuff like the compounding time effect of interest more accessible, I think.

Edit: If I could change one thing about pubic schools, I think everyone should get a chance to take stats or probability for free. It helps so much with so many areas of life.