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In order to check for shadows, we essentially want to check for what is visible at a certain point. Surfaces can be illuminated by a light source, so you can compute the illumination by sending a ray from the point to the light source. Sending rays from the light might hit objects that we don't see in the scene, which would not be efficient. When we send rays from the "eye" it is much more efficient since we only care about what we see.