when you pipe a set it's the same thing as calling union on each one, it creates a new set and puts all the variables in them:

so this:

  border = north | south | west | east

is the same thing as:

  border = north.union(south, west, east)

which is also:

  border = set()
  for border_set in [north, south, west, east]:
    for state in border_set:
      border.add(state)

it's just a new set with all the previous items in it. Since it's a set there is no doubles. for example, WA is in north and east, but only in border once, because there is no duplicates in a set.

to do it with lists you would need to make sure there was no doubles yourself. plus you wouldn't get the set operations later on, and would need similar loops to check things.

  border = []
  for state in north + south + west + east:
    if state not in border:
      border.append(state)

Sets are great any time you need to work with groups of unique things and see how they relate to each other. Since there is only one of each state, this is perfect use for them.

edits throughout as I thought of other things.