In the 1700s, barbers not only gave haircuts and shaves, they also pulled teeth, performed minor surgery and did bloodletting.
From Wikipedia : The origin of the red and white barber pole (often red, white and blue in the U.S.) is associated with the service of bloodletting and was historically a representation of bloody bandages wrapped around a pole. The original pole had a brass wash basin at the top (representing the vessel in which leeches were kept) and bottom (representing the basin that received the blood). The pole itself represents the staff that the patient gripped during the procedure to encourage blood flow.
From Wikipedia : The origin of the red and white barber pole (often red, white and blue in the U.S.) is associated with the service of bloodletting and was historically a representation of bloody bandages wrapped around a pole. The original pole had a brass wash basin at the top (representing the vessel in which leeches were kept) and bottom (representing the basin that received the blood). The pole itself represents the staff that the patient gripped during the procedure to encourage blood flow.