Flaps 15 landings are done to improve your approach climb angle. In any condition where you cannot meet the required approach climb limit flaps 30/40, you would do a flaps 15 landing. It is a common requirement at high altitudes in the summer in a -200. You do have to check your brake cooling limits as well.
According to our operations manual, flaps 30 is normal. Flaps 40 is at the pilots discretion, but recommended for tailwinds and wet runways. Also, a flaps 30/bleeds off landing is preferred over a flaps 15 landing if that improves the approach climb suffieciently.