It shows that affection is normal and healthy.
Affection is normal because we are a social species. We use touch to communicate positive messages of caring. Oh yeah, and touch feels good. Simple touch lowers levels of cortisol, a stress hormone. Because of this fun fact, watch out: Affection is essentially addictive.
It teaches kids that affection between adults in love looks different and is different than that between kids and parents.
Sure, you know the difference but, remember: They’re kids. How can they know there’s a difference between platonic touch and potentially-sexual touch if they can’t see a difference? That’s one reason kids express so much disgust when parents kiss; they are confused. Help them out.