Child car seat manufacturers are bound by very strict guidelines, and you may be surprised to find that some of the more expensive car seats don't provide any better level of protection than those which are more moderately priced. Many State laws require children to be in a safety car seat only up to age 4 or so, and many parents assume that children who are older than this will be safely restrained by just an adult safety belt.
This, however, is not the case. An automobile's seat belt system is designed for use by adults, not small children, and it usually will not fit a child properly. A booster car seat is designed to improve the fit of the vehicle's safety belt system across the child's body, for greater comfort and safety.