A booster sear does two things: It places the child higher so the seat belt fits properly, and it supports the child's uncontrolled forward and back motion should you end up in a collision or braking quickly.
Instead of the child's body alone being forced against the seatbelt, the booster moves with the child's body and disperses the energy, which results in a lower impact on your child's body and lessens the likelihood of seatbelt injury.
If you try to latch or secure a booster that isn't meant to be latched, it disables the booster's ability to support your child's body in a collision or when you have to quickly stomp the brakes. It seems odd, but when you consider it from a physics perspective, it starts making sense.
If you want your child to be better secured, there are manufacturers who have boosters with 5-point harnesses and latch systems.
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C. Lee