Why does the auto industry think that tachometers are necessary in passenger cars?

I’m an old guy and have been driving since 1948 or so. Back in the 70’s or 80’s automobile manufacturers began putting tachometers on cars. I’m not sure why there was a demand for this instrument but it doesn’t seem particularly valuable to me. The display on my current 2013 vehicle is as large as the speedometer and takes up quite a bit of space on the dash. I can look at it to see if the engine is running, of course. I see that my car idles at ±700rpm and I suppose that’s of some value. Perhaps someone can enlighten me on the reason for the demand.