When you ask an LLM to write some prose, you could ask it "I'd like a Pulitzer-prize winning description of two snails mating" or you could ask it "I want the trashiest piece of garbage smut you can write about two snails mating." Or even "rewrite this description of two snails mating to be less trashy and smutty." In order for the LLM to be able to give the user what they want they need to know what "trashy piece of garbage smut" is. Negative examples are still very useful for LLM training.