There are two reasons you would add placeholders.
1. A placeholder can be added when you don't have a staff assignment to fill a shift but you wish to publish the schedule. Once you publish a schedule with a placeholder on it the shift with the placeholder will be sent to the Shift Board as an Open Shift making it available for staff to pick up.
2. If you are using work assignments placeholders ensure if a shift assignment is delete your work assignments are not lost.
Example:
Megan was assigned a shift with a number of work assignments attached to her shift but then the shift assignment was changed from Megan to Janine. In the process of deleting Megan from the shift the system has no way to hold onto the work assignments because they were assigned to Megan so they get deleted when Megan is deleted off the shift. However, if Megan's shift assignment was first created as a placeholder the work assignments are attached to the placeholder and NOT the person holding the shift assignment. Thus, you won't lose your work assignments if you have to replace shift assignments.