Because even if you can't literally rattle off the stuff o to a 10th grade algebra exam, you remember the general gist and some of the details. I could not do a line integral right now but I know it exists, what it's for, and if I ever had a programming problem that was a line integral, I would remember it and be able to Google "line integral Wikipedia".
There is also, in my opinion, great benefit to just training your brain. Even if you forget all the details, I think someone who learned calculus and then forgot it is going to generally be in a better place to handle any general mathematics than someone equal who never learned calculus at all.
There is also, in my opinion, great benefit to just training your brain. Even if you forget all the details, I think someone who learned calculus and then forgot it is going to generally be in a better place to handle any general mathematics than someone equal who never learned calculus at all.