Why is it safer to use the tether?
Using the tether will decrease how far the child’s head will move in a crash by at least 4-6 inches typically. The less the head moves forward, the less likely it is to hit hard parts in your vehicle–like the door frame, the window, the back of the front seat, or even other passengers. The tether also reduces the amount of acceleration and how much force is applied to the neck during a crash.
In action: This video shows the same car seat with the same dummy in the same crash test… the only difference is that the seat in the foreground is using the tether strap, while the seat in the background is not. In a real car with a vehicle seat in front of you, you can easily imagine that the dummy in the background could hit his head on the back of the front seat or other hard structures because his head moves so much farther forward.