The novel is set in the traditional mode of satirical travel literature. Many other classic works use the same device, such as Chaucer's Canterbury Tales and Homer's Odyssey. Travel in the case of Gulliver's Travels gives Swift the opportunity to compare the ways of humanity, more specifically those of the English, with several other ways of living. Travel also keeps the story entertaining. It is not often that a person finds a book with four sailing journeys each interrupted by torrential storms, although one should remember that the Age of Exploration in Europe provided many stories of travels and discoveries of new lands and new peoples.
Your other questions require too much detail for this short-answer forum.