Car rental companies charge additional driver fees because rental vehicles with more than one driver are involved in more accidents and have a higher incidence of vehicle damage. By charging a fee for additional drivers, car rental companies can recoup some of their losses and hedge against the increased risk to their vehicles. The probability of damage to the vehicle decreases if the additional driver is the renter's spouse or business associate. This is why some car rental companies waive fees for these drivers (and usually don't require that their names appear on the rental contract).