The purpose of this line of questioning is to establish consistency in one's moral framework. Simply put, the process starts with the following question: what is the morally relevant trait that a human has and an animal lacks that if a human lacked that trait it would make it ok to exploit and slaughter that human for a product (or any variation of it). With this question we're seeking to find the morally relevant trait difference between a human and an animal that make it ok to exploit and kill animals but not humans.