You don't need to use it if you don't need the derivative. If you do need the derivative, then it isn't double the work because you would have needed to call a

You don't need to use it if you don't need the derivative. If you do need the derivative, then it isn't double the work because you would have needed to call a separate function for it anyways. Had I known about it at the time, it would have saved me a few headaches when I was writing a game mod that computed orbital intercepts.