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.
When I first started driving I inflated my mom’s tires to the number on the sidewall and I still cringe thinking about how I probably almost killed myself with an exploding tire. I got home and I asked my brother if it’s supposed to take that long to fill up and he freaked out and we let the air out and he taught me the very valuable lesson of checking the placard.