Build a simple wooden fence (or an elegant iron fence) and we are faced with design choices…some of which we may not even realize at the time. The size and spacing of the pickets of a fence have a profound affect upon the overall outcome. I chose these fences because both are lovely in their own way and highlight key differences.
The first photograph is a stained fence that has small pickets spread apart (as I recall) 6″ on center. The effect is that the garden is viewed as a continuance from outside to inside of the fenced area. In addition, leaving the fence stained allows the fence to further blend into the garden, especially if the occasional vine is included. The designer (me, in this case) wanted the fence to remain as subtle as possible while still defining a space.
The photograph below (white fence with closely-spaced pickets) illustrates how the fence can act as a visual barrier to the inside space, more clearly defining the interior and exterior space in the garden. In addition the fence (especially considering the white color) becomes an effective backdrop to the foreground plantings. Both the iris and rose are brought into a sharper focus against the white fence, whereas the same plants placed in front of the fence in the top photograph would tend to blend into the landscape.
Neither fence is superior nor is there a correct answer, as design is subjective in many areas. The point is that if one chooses to build a fence being aware of these design considerations will help the designer or landscape architect in their design choices.