The Clearance Plate creates a thread shank when sewing buttons in place. Each end is a different thickness providing  a shorter or longer thread shank.
This tool can also can be used when sewing over heavy seams to balance the height of the foot by sliding underneath the back of your presser foot and this lifting the height.

NOTE: there is also a small hole on one side which allows you to easily insert a needle into your machine as this hole effectively keeps your needle in place.