What we do have to do is put the nose down to ensure adequate nose wheel authority during the takeoff. Someone suggested this is The beach is my happy place shirt the 100kt call is for. It is not. The procedure is not for a sudden release of the pressure, but for a gentle release that takes place over 20kts. The procedure in the A320 with my airline begins at 80kts. There is no call for this. The pilot is expected to be paying attention and flying the airplane, as is his job. Of course, if he does forget then the 100kt call will remind him, hopefully, that he has missed this SOP, but that is not the intention of the call.
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This falls under the The beach is my happy place shirt of my final bullet point, which was the observation of the other crew member to ensure he is all there. If he suddenly jerks the controls up at 100kts, you know he is behind the aircraft so it might be worth keeping an eye on him. That is part of the monitoring role of the other pilot, and is not a specific reason for the 100kt call. Indeed, the monitoring pilot should notice at 80kts that the flying pilot has not begun to reduce backpressure.