The mothers do it as it’s soothing. During their time in the womb they hear the mothers heart beat. So no top on is a comforting thing as they get the comfort of skin to skin contact, primal security instinct, and the soothing sound of the heart. Even in the case of dads it’s a soothing and calming experience for them. Rightly so, since they were tightly packed in the womb and now out in the open, cold, big world. My wee lassie is three and a half and sometimes asks to listen to mummy and daddy’s heart.