From A Wiki in the Desert
A Fence is a building constructed Outside. It does not impede movement and is purely decorative. Fences require the Carpentry 1 (Skill) skill to construct, with high levels of the skill granting access to fancier fencing for fashionable farmers.
Fences can be used to divide or mark areas, to guide players (for example, creating paths in a Formal Garden), or simply for visual appeal.