[Calculus] Why do I even need derivatives?

I'm not a math nerd but I do realize that I need to understand it and that I probably will use it in my professional life. So I learn it and I think I more or less get HOW to calculate derivatives and how to write these calculations so they make sense. However, I still don't know and understand WHY do I do that, what is the whole point and use of derivatives. Can someone explain it to me? I know derivatives are used in e.g. physics but I'd like to know WHY? What's the purpose of differentiating a function?