One of the principal goals of satire is to point out various aspects of the satirist's society that need correction, and each of Gulliver's voyages, Swift manages to find some aspect of culture and/or politics in 18thC. England that needs improvement. A well-written satire allows the writer to identify problems in his fictional society that mirrors problems in his actual society but without overtly discussing those real-life issues.
In Gulliver's first voyage to the island of Lilliput, for example, as Gulliver begins to observe Lilliputian society and politics, he finds problems in Lilliput that have counterparts in England. When the Emperor of Lilliput picks ministers for his government, he chooses them not on the basis of their ethics or political skill but on their ability to walk a tightrope successfully, an slightly veiled criticism of George I's choice of counsellors and ministers based on their family connections rather than their skills in governance. In another instance, the Lilliputians are split into two camps based on which end of a soft-boiled egg they open first, the small end or the large end--a dig at the constant conflict between Catholics and Protestants in England.
In the land off Brobdingnag, as the King hears Gulliver's description of how wonderful English society and politics are, and how advanced people are in Gulliver's world, the King, especially horrified at Gulliver's description of European warfare, concludes that Europeans are "the most pernicious Race of little odious Vermin that Nature ever suffered to crawl upon the Surface of the Earth." Considering how proud Gulliver is as he describes England, this is a devastating response because it comes from a being totally outside European society who should have no particular agenda in evaluating European society
The third voyage, to Laputa and other islands, allows Swift to satirize the misuse of man's natural talents: he finds a group of people who spend all of their time trying to accomplish the impossible (and the ridiculous)--trying to build houses from the roof down; attempting to soften marble. Swift, who agreed with Alexander Pope that the "proper study of mankind is man," is taking aim at the numerous projectors in English society who spent their time and others' money to find methods of doing things that had no application to the bettering of man's condition.
The rational horses in the fourth voyage, the Houyhnhnms, who pride themselves on logic, reason, contemplation, control of their emotions, allow Swift to comment on the nature of man, perhaps the most important aspect of his criticism. Man, according to Swift, is controlled by his baser emotions rather than his intellect, and that leads to all of the evils--war, jealousy, general violence--tht keep mankind from progressing. He uses the Yahoos, servants of the Houyhnhnms, as the vehicle for his criticism, and every reader understands that Yahoos are men. By accepting the premise that everyone has a Yahoo in him, Swift is hoping that men will then consciously try to eradicate the vilest parts of their nature and become more like the Houyhnhnms.