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.
Only on holidays. If I pick up some furniture, I don’t bother, but when I’m driving for one or two days straight with 4 people and luggage in the car, yes definitely. It helps that I can check/adjust my tire pressure where I wash my car every two weeks. I’m really particular about the handling of my car and the tires I drive on, so I’m always on top of the correct pressure.