For what reason(s), do authors give characters deformities, scars or other physical markings in literature?

There is a certain amount of "otherness" to characters who have visible abnormalities. They cary with them emotional and physical baggage that other characters react to. Characters wit deformities, like the creature in Frankenstein, often have to interpret the world on a very isolated level. They must come to grips with their "otherness". They also see the world and themselves in unique ways.