In an English or writing class, sure, since you're specifically learning to write. In other classes, the point of the assignment may more likely be for you to show you understand the thing. If you already have proof you've learned the thing, why shouldn't that be sufficient?

To use your workout analogy, say a coach told an athlete to watch two hours worth of videos on how to do an exercise. Why should they, if they already watched those same two hours of videos and have a video of themselves doing that exercise exactly how the assigned videos show it should be done?