The call isn't for the benefit of the Air Trafficker alone, it is to help others plan their circuit. For example, if I've just climbed away on take-off and have heard someone else call 'downwind to land', then I may need to extend upwind to ensure that I don't get balked on the approach by an aircraft still on the RW after landing if I plan to fly a touch and go.
Aircraft in the visual circuit should plan their own activity without interference from the visual controller. There is nothing worse than being ordered about by some well-meaning controller who does not understand the effect of wind on slow aircraft, for example.
And as for those who think a 'visual straight in' takes priority over traffic already in the visual circuit.... I was Duty Dog at Benson once when a Queen's Flight 146 announced he was straight in to land. "No he isn't", I told the visual controller, "Tell him to join on the deadside as there are 3 aircraft in the visual circuit ahead of him!". Which he did; when he tried to whinge to the CFI afterwards he was told to go and read the FOB!