The proper name for a meter that reads RPM is a Tachometer. In a car, the RPM is the engine rotating speed. Since the transmission has multiple gears, the speed of the engine is not the same as the speed of the car. Different gears are needed because internal combustion engines have a RPM range where they produce the most torque or efficient, smooth cruise for best acceleration and overall performance and economy.
For many people, who grew up driving automatic transmission cars, the tachometer is pretty useless. Especially like me, whose first car was a GM car with a “Powerglide” automatic 2-speed transmission that had 2 forward gears and shifted to high gear at precisely 35 mph no matter what. At least I noticed.
Its most useful for those who drive manual transmission cars, helping you decide the shift point, although you could also claim that those could be driven by sound alone, the sound of the engine speed being the aural clue.
However for modern automatic transmissions involving 6 or 7 or more forward gears and sport or economy modes where the gear rations are programmable, the cars computer calculates the best shift points depending upon whether it senses you are driving grandmotherly or aggressively.
Still, its useful information to know how your engine is performing. Both of my current automobiles have either paddle shifters or semi-manual positions in the stick where you can shift through the 6 or 7 forward “gears” manually (no clutch). The tachometer is useful for driving authoritatively or economically.