When your child is aged between 12 and 15+ years, most of your jaw growth has already taken place. The permanent front teeth and the first permanent molars have been in place for more than 2 years, and the rest of the permanent teeth will be coming through within the next 2 - 4 years.
Because there is not enough time at this age to correct jaw shape with normal myofunctional treatment, and because the jaw shape should be corrected before the permanent teeth come through, a booster is needed.
It is best to gain space by developing the jaws when you are still growing. Correcting jaw shape quickly allows more room for the teeth, and myofunctional habits are more easily corrected.