Walls are good. Our ancestors built walls to keep wild beasts from carrying off loved ones, and keep dinner from running away. Walls successfully defended the ancient city of Troy until that nasty Greek gift showed up. Walls in a grain silo contain the grain and keep it from spilling out. On a submarine, walls keep the air inside and the sea from spilling in.
Walls divide things.
While the most famous such division may be the Berlin Wall, there are more common reminders. Walls keep the general public out of exclusive country clubs, and keep visitors away from the aliens in Area 51. Walls do both good and harm. While expressways may be good, allowing travelers to move from Point A to Point B with greater speed, they bring with them increased vehicular noise. What was once a pleasant drive with pleasant scenery is transformed into a frantic dash between two walls, with only a narrow ribbon of sky for a view.