Every car is different and may require specific air pressure for your tires.
This can be found on the frame of your driver’s door. This plaque will also tell you the recommended tires you should keep on your car per the manufacturer.
This usually applies to your everyday driver that probably doesn’t have custom tires or rims that would null the recommended pressure for the car.
Edit: WHY you should know! (I forgot to add thank u @_MoveSwiftly!) So keeping your tires inflated is important because a) it keeps you safe. Properly inflated tires will have better stopping power B) wear and tear on a tire is lessened and wear and tear on other aspects of your vehicle that require good balance like shocks and brakes. c) in most cases having your tires inflated to the proper psi/bar will help you hit the MPGs, horsepower and acceleration advertised for your car.
The pressure on the sidewall is the max for that tire, not the recommended psi. (Remember, the tire fits on many different vehicles) The tire placard on the vehicle is calculated based off the gross vehicle weight and the contact area of the tire (with the recommended size) to get the best wear and handling. If the tire placard calls for more than what is on the printed sidewall then you have the wrong tires for your vehicle, either load range or size.
Wrong load range is dangerous. Aftermarket size and you have to do the math yourself.
Tldr: Air your tires to what the placard says. Not what the sidewall says. If the tire placard says less than the psi on the sidewall is OK (preferred), above that psi means you have the wrong tires.