To me, there are at least three reasons that I can think of:
- It makes for a more interesting piece of writing. If you were just to write an essay about what you thought was wrong with government and religion in England, people would not be too likely to buy it. Make it into a fun book like Gulliver's Travels and more people will buy.
- It makes people think. If you read the book and you have to think about what it is saying (you have to figure it out) you're more likely to really remember and take it to hear.
- It might be less likely to make people angry. It doesn't directly attack things and so people who like the institutions might not get as mad.