Why do F1 racing drivers get weighted after a race?

According to F1 regulations for the 2012 season, cars must weigh at least 640 kg (1411 lbs) at all times. This includes the weight of the driver and his helmet.

After a race, the weight of the driver and his helmet is added to the weight of the car and that total weight must be at least 640 kg/1411 lbs.