When you drive your car on a flat tire you risk damaging your vehicles rim and even risk damaging the rear end or front end depending on where the tire is flat.
If the flat tire is on the front you may have difficulty steering the vehicle but you can drive very slow on the flat tire if it's your only option and it's not safe to stay on the highway or you're just a short distance from a tire repair shop or town.
If you have a spare tire and can safely change the spare tire on the road then do put the spare on instead of driving your vehicle with a flat tire to avoid damaging the rim or other parts of the car.