A place holder allows you to enter it to a contest that you want to enter at a later time.
Once you have created the set, you delete the place holder and enter the set to the contest.
This allows you to mark the contest and find it quickly. You dont have to spend time scrolling through the contests looking for it.
In addition, you can hide a required item behind or with in your place holder.
This will allow you to enter a item required contest.
See other Polyvore's creations: http://www.polyvore.com/place_holders_talented_polyvores/collection?id=4311609