There are 2 lore reasons for scars on guardians that we know of. 1. Ghosts can only revive up you to the state of your first death. Meaning any scars you obtained in your first life would exist on your guardian body as well. 2. Ontological attacks can scar guardians. Ontology is the state of being or existing. Ontological attacks are attacks that question our state of being with the laws of realty/the “logic” of the faction we’re fighting (Hive Sword Logic or the Vex’s “Pattern” Logic) Most of these attacks are capable of wiping us from reality, but some ontological attacks can also happen on a smaller scale to cause scaring. For example a Hunter travelled into the Black Garden in a vision. There he encountered a flower in the shape of a ghost that spoke to him (we now believe this to be another manifestation of the darkness) at the end of their conversation the hunter picked a flower, and was pricked by its thorn. When he awoke the wound from the thorn was still there. It never healed.