Not only is the requirement to advise PRIOR to descent to enable ATS to review any known traffic, but to let the VFR chap 500 ft below you that you are about to start down! Another reason why you should say your position with the same call. ATS know where you are (usually) but the VFR below do not.
Ah, but the VFR should be applying the simple rules of See-And-Avoid as one luckless Dickless individual would have it....
ATS should know where the guy is. Location was never a part of the call as far as I was aware (way back then in my GA daze....) but what did I know???