It's a dominate/submissive thing.
Whether its in the bedroom or not, if you call someone mommy, you would be saying they are nurturing you and loving you. If you call someone daddy you are saying they are taking care of you.
So for the guys that like it, you're telling him he's taking care of you sexually.
But it definitely is cause the girl is acting submissive to him when she says it. It's like saying 'You're in charge.'