Mostly becuase it sounds so deliciously rude, but isn't.
For those who enjoy useless infomation, "1. An arrangement of things by fives in a square or a rectangle, one being placed at each corner and one in the middle; especially, such an arrangement of trees repeated indefinitely, so as to form a regular group with rows running in various directions."
Now if I could only find a way to work it into conversation at least once a day I would never be sad again.