One of the reasons why it is not required to report descending when you are
cleared for the visual approach is because it is quite plausible that you
will be out of radio contact when you begin your descent. When the
controller says "Cleared for the approach" not only is he assuring there are
no airplanes on your intended course to the airport, but also he is
transferring terrain avoidance responsibility solely to you, whereas
previously he shared this responsibility with you.

--
Richard Kaplan, CFII

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