Some people try to optimize their FF program, and each program requires a different set of codeshares or airlines. Some want to get exactly 16 segments. Some just want to visit some fun destinations. Some try to optimize the cost requiring an Oslo departure, or similar, and to reduce the surcharges by selecting flight segments optimizes for cost (as opposed to points). Some want to fly specific routes or airlines or airplanes. Some people skip the AA RTW desk in an attempt to train other airline reservation staff.