From about the 12th century, knights wore long, flowing surcoats, frequently emblazoned with their personal arms, over their armor. These usually extended to about midcalf, had slits in the bottom front and back, thus allowing the wearer to ride comfortably and were either sleeved or sleeveless. Historians believe that the practice of wearing white ones was adopted during the Crusades, their main purpose of reflecting the direct sun, which overheated the armour (and the soldier inside) – although it may be argued that here its color would have been of little help; while in poor weather they helped keep rain and the muck of battle away from the easily corroded mail links. The surcoat displayed the device of the knight (origin of the term “coat of arms”) thus identifying him; which in turn, combined with the increased use of the great helm (late 12th century, early 13th century) became an essential means of recognition. Indeed, some historians cite this as one of the reasons behind the spread of heraldry across medieval Europe.
Source: Surcoat – Wikipedia