Answer:
Well, when i use a placeholder sim it's usually for a story or a sims challenge. in most cases the "Placeholder" is just there because you can't have a sim without being related to an adult sim. so for instance,
you make a female teen and her mom, and then you make a male teen but you don't want them to be siblings. you can make a placeholder sim be the male teen's parent so he is not related in anyway to the female teen.