People use fix-a-flat to inflate tires that have gone flat or need some additional air, and they do not have a readily accessible means to fix or inflate a tire. It is usually recommended that people not use fix-a-flat any more especially if they have a tire pressure monitor system on their car since the product will ruin the sensor. You may also ruin the tire beyond repair if you use the product. Plus the repair tech taking care of your car will not be too happy to have to clean the tire rim.
However, not all canned air inflation products ruin the tire pressure sensor. If you buy only products that specify that they are safe for tire pressure monitor system cars, that product should be safe to use. Still, even this product is not great and should be used as a last resort, which may be less costly and quicker than road service. The tire ultimately should be repaired or replaced, and not driven as though it was fine. Also, today's products of this type are nonflammable.