The Inline Fabrication Ultramount Flush Mount Quick Change Base Plate Spacer is required if you are not routing the flush mount base plate into your bench. The flush mount base plate was designed to be routered into your bench to provide a nice flat bench top. However, If you don't want to route your bench top (i.e, mount it directly to the surface) you must install this shim plate to raise it up a touch to allow the quick change top plate tongues to drop in and out. Note:You still need the flush mount quick change base plate (sold separately).