This site's own five-habit list includes this specific fourth point: choose the 6-bottle package for best value.
This site describes it directly: the lowest per-bottle price, with free US shipping and 5 bonus guides — worth reading these three specific benefits together as the stated case for this tier.
This site's fifth habit recommends giving it 30 to 90 days — the 6-bottle package's 180-day supply comfortably covers that window with room to spare, worth reading both habits together.