An NPC wears clothes according to their status and position in the world. A blacksmith such as Adrianne Avenicci would wear a blacksmith's apron while most merchants would wear normal clothes. Couriers usually wear belted tunics and beggars, such as Brenuin, usually wear ragged robes along with a ragged cap.
Fine Clothes are usually worn by the noble men, for example Olfrid Battle-Born. The Jarls of Skyrim, such as Jarl Balgruuf usually wear noble clothes. The Emperor wears unique clothes, that can also be found in his dresser aboard the Katariah.