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.
You should also know that normal driving will raise the temperature and therefore the pressure of your tires. This means if you’ve been driving for a while and set your your pressure to exactly what that placard says, you’re most likely going to have low pressure the next morning. Tire pressure should be set when cold. If that’s not possible, add about 4 psi more when you’re at the pump. The next morning you can use any cheap pressure gauge to check and let some air out to correct if necessary.
My cars have always had a message saying “+0.3 bar when hot” or something, so I’ve just done it according to that.
You never get the absolute exact perfect number anyway, because temperatures vary, loads vary and speeds vary. I just like to keep it somewhere below the empty car pressure and loaded car pressure, because sometimes I’ll have passengers and loot, sometimes I won’t.