Satire is a technique employed by writers to expose and criticized foolishness and corruption of an individual or a society by using humor, irony, exaggeration or ridicule. They use fictional character to stand for real -people to help expose and condemn there corruption.
It could be just to entertain you the reader. It could also just go with the character and the tone of the book. For example, if you made a novelization of the TV show friends you would us a lot a satire.
