Its perhaps only shape I can remember which can be used to form patterns like: And believe me that's important as its only the patter used while making communication areas called cells. Even bee hives are known to have the same shape. read more
Their shape is a direct consequence of a basic interaction, and you get very regular shapes like hexagons. For macro stuff like bees cells, balls stacked together, etc : it is just the more convenient way (eg requiring less energy) to stack similar stuff together. read more