Great question Hopeful!
The purpose of the Columellar Strut is to support the nasal tip after rhinoplasty, to prevent it from drooping. The Plumping Graft, on the other hand, is meant to blunt or otherwise change the nasolabial angle (the angle formed between the columella and the upper lip). Plumping grafts are essentially camouflage where as columellar struts are structural.
A columellar strut, if fashioned appropriately and sutured in appropriately, can increase the nasolabial angle, but this isn't its main purpose. It is generally not used for columellar retraction. A better option for columellar retraction is a plumping graft, or perhaps a caudal septal extension graft.