I am taking an ASL course to help my youngest daughter who has unilateral hearing loss. In the learning process one of the books I used explained how learning the manual alphabet is a very good thing to learn even for hearing children. It is very good for hand-eye coordination as well as manual dexterity. It also gives the child another way to communicate which can be a huge bonus especially if it is a setting where your child is supposed to be quiet and they talk non-stop provided it's not like my son's friend who just talks to hear himself talk. There are some kids that are autistic that are not verbal and that is not my son but sign language whether ASL or some other manual form of language has helped these families tremendously.
Another bonus to it is depending on how your child learns this might be a way to help them improve their reading and spelling if they must have movement.