We ask this question for a few reasons--to see how the candidate engages, quickly thinks on his or her feet and fits into our overall company culture based on the answer. Even if the joke doesn't make any sense, it's about the presentation and ability to adapt to a curve ball that we're looking for. A silly question, such as asking candidates what fruit or vegetable they would be and why or what kind of animal they would be and why can help us find superior candidates that best fit our culture.