Semolina is more coarse and won't stick to the pasta as much as regular flour. Semolina is great to prevent sticking but it won't 'cake' onto the pasta like flour so it keeps the pasta lighter and easier to digest without so much of the added white flour. It keeps that pasta later as an end result. When the pasta is ready to cook it won't have an excess layer of raw flour that will cause the water to get cloudy looking. It also doesn't make for the right kind of pasta water to save as a binder for the sauce.