This link shows a handy device to hold a carrier safely in place on a car seat:
Amazon.com: Kurgo Carrier Keeper - Argyle: Pet... It is just the straps that help hold the carrier in place. It is a much safer for when you travel. I think the car seats are great for around town driving but if you are traveling any distance this is true protection for your pet. I know many of our babies are spoiled by other ways of travel. Gracie doesn't mind traveling in a carrier as long as she can keep track of what is going on with her people.
As Yorkietalkjilly said it protects in case of flying objects. If a car gets hit by another car things, people, pets, and glass flies!