Unlike a wall, which is an opaque structure, a fence can help to establish a boundary while still keeping things feeling airy and open. Even solid panel fences have a more open feel than walls do — especially if you leave a few openings here and there to pass through.
Fences can be used to mark specific boundaries, like the corner areas of the property, without entirely enclosing it. This can be very helpful in surveying the property. If you ever sell, a fence it can assist in presenting a clear view of the boundaries of the property.