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.
YSAK that getting new tires doesn't always line up with the PSI rated by the vehicle manufacturer and will have the rated PSI listed on the wall of the tire. You can also request the documentation on the tires purchased from the shop you got them from.
Edit: As rightfully pointed out in replies to this, yes the markings on the tire wall as typically the safety ratings for the tire itself and the correct PSI is a more complicated relationship between the vehicle, the size of the wheels/tires, and contact area of the tire for optimal traction and fuel economy. The vehicle placard will be a good value in most cases, except when the tire marks a max pressure lower than your vehicle's recommendation. Always consult with the shop if you have concerns.
