This is one of our favorite, multi-purpose behaviors that is a foundation for other behaviors.
We use hand touch for greetings, moving your dog through space, into an unstructured heel, redirecting, getting attention, or just for fun. For reactive dogs, we use a hand touch as an alternate behavior for barking and lunging on leash. For fearful dogs, this behavior gives the learner warm fuzzies about having hands nearby.
The hand touch is often used for short distance recalls. We like to do these fun recalls to "ask" our dogs to close the gap of space between us and them instead of us walking to them.
Using positive reinforcement to teach this behavior also helps dogs feel good about their caregivers as a bonus side effect of training this important cue.