Well, historical appropriateness for one thing. If you're trying to play D&D in a poor simulacrum of medieval Europe, and you notice that innkeepers in medieval Europe often had wives, then you give your innkeeper a wife. You don't need to justify it on any other grounds.
But if you give your innkeeper a Siamese twin, or schizophrenia, or a gay lover, or a flamethrower, you're deviating from historical patterns and the players will presume you have a reason for it.
Because everyone knows that there were no Siamese twins, schizophrenia, gay lovers or flame throwers in medieval Europe.

