It's to maximize the edge. You will get more production out a curved line than a straight line.
Permaculture is fascinating in that it teaches us to look at nature's non- linear systems, and understand the basic underlying patterns of natural phenomena.
In the example of an oak leaf we find a pattern that allows for maximum accessibility and minimum amount of path. Maximum edge means maximum use of pattern.
Planting in circles instead of rows is a practical use of this concept.