One of the main benefits of a high-back booster seat is that the upper portion features shoulder belt guides that position the vehicle seat belt so that it doesn't creep up onto the child's neck. The shoulder belt guide also makes it difficult for children to slip the shoulder belt behind their torso or under their arm, both dangerous mistakes. Some high-back booster seats have a deep top portion that offers extra side-impact protection, but more commonly the wider side portions merely offer a comfortable place for a sleeping child to lean on.
High-back booster seats are also needed if your vehicle does not have head restraints (commonly called headrests). In this case, the high back of the booster acts as a head restraint, preventing whiplash-type injuries in certain types of crashes.
Your child will have outgrown a high-back booster by height when the tops of the child's ears are above the top of the booster shell. Many booster seats offer an adjustable-height back to accommodate a growing child, and many also can be converted to a low-back booster seat.
Booster seats must be used with a lap/shoulder seat belt. Booster seats should never be used with a lap-only seat belt.
When buying a high-back booster seat, look for a height-adjustable back, side-impact protection and wide shoulder belt guides that won't pinch the seat belt or allow slack to remain in the belt.