Why are such light limbs necessary or better?
1) Just pulling the string back any way possible makes it impossible to learn correctly. And for kids starting out, almost any bow is too heavy. They fatigue quickly, and will develop poor technique because it becomes a fight just to hold the string. These light limbs allow kids (anyone, really) to work longer in a training or shooting session without undue fatigue. This just makes it easier to get consistent, which is the end goal – proper technique done consistently.
2) If you are wanting your child to train for competition, just from life, there is a discrepancy in relative strength needed to shoot proper technique. New muscles are used and must be allowed to strengthen; they must catch up to other muscles that are stronger. No matter how strong the person is, some muscles are strong, others must be built through training. With a heavier bow, the presently stronger muscles will continue to be used because the new muscles needed are not strong enough at the beginning. Lighter weight starts the strengthening of the weaker muscles.
3) Really light limbs are necessary for kids and teenagers, but also for most adults if they are serious about learning good technique. Training in archery has become so specific that using too much weight at the beginning results in a certain level of proficiency, but not what is needed for long-term success. New shooters must be able to practice proper technique without fighting the bow to learn properly. Ideally, before using a bow, a person also spends time working with a piece of stretch band or tubing in front of a mirror, which is even lighter weight.
4) It is so much easier to learn properly from the beginning than to allow a person to just shoot, then try to change the established habits. Unlearning and relearning takes so much more time!