What was the purpose of the trip in chapter 19?

After they get back on the road, the narrator asks if that was part of a homework assignment. The mechanic answers yes, it was part of it. His assignment is to make four human sacrifices that evening and then pick up a load of fat. The mechanic asks the narrator if what he said about his job is true. The narrator replied that it is true.

Next, they go to the medical waste dump in search of liposuctioned fat. The plan was to steal the fat people spent a ton of money to have sucked out and then use it to make soap to sell back to the people who paid to have it taken out.