The goal of every rogue is to deal the maximum possible damage. In order to accomplish that goal, we have to figure out the best stat and rotation configuration. Discovering the best stat and rotation configuration would take weeks of work if done empirically. We can't simply apply brute force to make the issue go away. That strategy only works against mages and hunters.
In order to quickly determine the optimal configuration, many intelligent rogues have worked on modelling our damage and rotations mathematically. As WoW's mechanics and systems have evolved, so have these models. While they're getting more and more accurate at predicting rogue performance, they're still not 100% in sync with what actually happens in-game. Rogues are a class played with a keyboard, not a pen and paper.