Because the answer might give some insight into a person's personality. Which is a the main thing a good interview is supposed to try and probe. If you make it that far then they should already have good reason to believe you've got the necessary skills.

So, in a sense, you're right. It's a good question because their is no correct answer. An interviewer who's asking many questions that have correct answers is an interviewer who didn't exercise due diligence during the pre-interview process.