As long as the transmission or clutch isn’t slipping, having both a speedometer and a tachometer is probably redundant. You can mark the tachometer for the speed in each of the gears and you can mark the speedometer for the rpm in the gear.
Just to repeat a few that I agree with…I think it’s essential too as the owner’s manual point’s out, for appropriate down shifting w/o damaging the transmission for auto or manual.
Keeping track of idle speed during warm up. I like to wait til it’s settle a little before "I’m out of here."
It can also detect, like a warning light, poorly functioning transmission and encourage light throttle application to induce an automatic to shift as soon as possible when accelerating.