Border demarcations are one of the more obvious uses of "line"....
David Beaulieu
Border elements such as fences define a property, turning it into a pleasing "nook."
But do you understand why? Let's consider the issue in more depth.
Borders play an important role in landscape design. Whether we're talking about the whole yard or just a planting bed, a defined area catches the eye more readily than does one that is "open-ended."
Don't believe me? Take a drive, and make it a point to note which landscape designs impress you the most. I can almost guarantee you that you'll find properties set off by hedges, fences or stone walls to be more attractive than those that aren't, all else being the same (similar houses, similar plant material, etc.). Such well-defined landscapes just look more "finished."