When a cars tire is low more of the tires surface area will touch the ground when it shouldn't.
When too much of the tires surface area is touching the ground it increases friction which can cause your tires to overheat and wear out too quickly.
If your tire overheats too much then you risk tire separation and a tire blowout causing a nasty accident that can cause injury to you and/or your passengers and may even prove to be fatal if the crash is severe enough.
If your tire is low you should not drive too fast until you get the tire aired back up again and check the tire for leaks and have it fixed.