According to Cognitive Evaluation Theory, people can have their intrinsic motivations grow when their action is rewarded with a sense of competence, feel that their action was the result of autonomous decision making, and by how much their actions fostered a feeling of social connection. Intrinsic Motivation – selfdeterminationtheory.org

Being properly praised for achieving a task can certainly check all of these boxes.

But why? I don’t personally have many answers yet, but a potential answer can be assumed from an evolutionary perspective. The ability to grow intrinsic motivations is certainly something that nature would select for, due to its benefits to productivity, survival and social cohesion. The Emerging Neuroscience of Intrinsic Motivation: A New Frontier in Self-Determination Research – PMC (nih.gov)

Looking at it from that perspective, your brain giving you a reward whenever you do something that benefits others, or that gives you value in some way, of your own volition, competently, and then restructuring in some fasion to make sure the behavior occurs again makes a ton of sense.

Praising a child’s actions in order to change their behavior is a simple and effective tool for ensuring behavior is repeated in the future.