Why is the setting important in Rikki-Tikki-Tavi?

That's quite a difficult one. The mongoose lives in an Indian villa which would have been very familiar to Kipling. It's his familiarity which makes it real and vivid to us even though our own lives are so far removed from his. Secondly, for all the anthropomorphism of the story, the animals are behaving in real ways. Mongooses do kill snakes and cobras do attack. The setting provides the stage for the action to be credible and exciting.