What does the setting help establish in a piece of literature?

Setting can be of no importance or of great importance or important only in itself.

The setting of 'Star Wars' isn't really important to the plot as the rebellion could have happened in any setting: western, medieval European, wall street, Aztec Kingdom, etc etc etc. However the setting is still important in-itself as lots of people showed up to see it simply for the special effects.

The setting of Count Dracula or Frankenstein or Moby Dick however is important as the time, place and morals of the time are vital in driving the plot of the story as what happens would not have happened (exactly) unless it were in that time and place.