Fences are used to keep things out (or in). They are used to mark boundaries and protect property and privacy. While white picket fences seem old fashioned, they continue to be popular.
To me they represent the idea of a happy family, the type you'd see on a sitcom. In reality, anybody can have this fence. I've seen sad looking picket fences and others that have been maintained. Some protect wayward yards and or guard homes that look like they should be in a 50's era photo album.