I'd argue that it's a feedback loop - namely, that the driver wants to know speed because that way he can drive to optimise it around the track. He knows what s

I'd argue that it's a feedback loop - namely, that the driver wants to know speed because that way he can drive to optimise it around the track. He knows what speeds he needs to be at, at specific points on the track, to optimally corner, etc. Not to mention that the precise limits of what speed can be achieved where will change as track temperature and conditions change. By giving that to the driver, they can use their intuition to tune their optimal lap performance.