The point is to isolate variables. If you want to see whether X might be causing Y, you can't just do X in random environment while A, B, C, D and Q is happening, and claim if you observe Y that was because of X. You have to isolate X and establish a link between environment with X producing Y and the same (to the extent possible) environment without X not producing Y. Then you can make a step (still not enough to be sure but at least to start suspecting) that X is causing Y. Of course, it could turn out that X is causing Y only when A is present but B is absent, and then your experiment will be a failure. Or maybe Y just randomly happens and you had back luck to land on it exactly when you did X. Nobody said it's always easy :)
