Safety - In case of a sudden stop or accident, your dog will not be thrown off the seat into another passenger, the driver or against an object such as the window or seat.
Security - The instant you stop the car to get out, your dog may try to follow you, even in places that are not appropriate such as a gas station. If your dog is likely to run around or does not listen well to your “stay” or “come” commands, they could get hit by another car or even refuse to get back in the car.
In the case of an accident where you are injured, you may not be able to control your frightened or angry dog. A seat belt keeps them restrained to enable emergency personnel to help them.
Behavior - If your dog is likely to be moving around a great deal while you are traveling, and getting into things they shouldn’t be, or excitedly climbing all over the car unable to sit still, they may be uncontrollable while you are driving. A seat belt will help your dog understand that good behavior is required in a car.
Comfort - A crate is one option for car travel with your dog, but that could be very uncomfortable on a really long car trip. A seat belt designed for dogs allows him to see what is going on and enjoy the travel experience, and still allows for limited movement so he can shift position and be comfortable while you are driving.
Distractions - While you are driving, you need to be able to focus on the task at hand, not on an excitable dog or one who wants to have your attention at an inopportune moment. A wiggling, climbing dog is very distracting for a driver, and dangerous. One little distraction at the wrong time can cause an accident.