A sissy bar is also known among riders to be called a "sister bar" or "passenger backrest." This addition to the rear of a bicycle or motorcycle for your passenger to be able to lean back and relax a bit while enjoying the world go by. As an alternative, however, it can also be used for luggage or other add-ons for your motorcycle.
These days many motorcyclists use or install a sissy bar for their passenger's comfort giving them something to lean back on for the long ride. More often than not, however, the bar is used to hold sissy bar bags. A series of luggage pieces that hold a bunch of gear and other important items. Some even place a flag holder on their sissy bar for a different look.
Whatever your preference, the motorcycle sissy bar has been around a while, and seems to almost be a must-have for motorcyclists.