No one is forcing you to. If you already know it, and can do it easily and well, why would you buy it?
However, if you need help with a sleight, Jason England's tutorials are probably the most in depth visual tutorials out there, and he has a ton of experience (obviously, The man is Jason England! One of the best in the modern business.), so he can easily walk you through the dos and don'ts.
So you may know the straddle pass, but if you want to or need to make it better, go to his tutorials!
So for example, I probably wont be buying his Classic Pass, not because I think mine is perfect, but rather because I think I have developed it well enough to perform it whenever, and I do. I am still developing it, but I think it is far enough along that I can focus on learning other things, such as the side steal, something I have never been able to do before and need help with, and thus will probably be buying his tutorials.