It is actually a perfect testing question of attitude: Does the candidate get a sneer? Do they answer with attitude, or with zero creativity or interest? "I work too hard. I care too much."
Awesome, they've weeded themselves out as no hires. I thank sites like Reddit and HN for engendering such a sense of attitude among so many that it allows you to filter them early in the process: No need to get to the technical questions if you can't answer a simple question of a professional peer.
It is not a difficult question. Further, people who say that it's to "test for self-awareness" don't get the point of it. You can actually ask almost anything of an interviewee and discern knowledge about how they think, how they reason, and how they communicate. This is just such a question, with loads of information delivered by the response. Because when I'm interviewing we're looking for someone to work closely with us -- minus the attitude -- for hopefully years. If we just want to ask technical questions we'd hire a consulting company.