Why are candidates asked to "list main weaknesses"?

Jane asks: Why are candidates asked to "list main weaknesses"? The question itself is intimidating, instantly puts people on the defensive, and encourages people to lie. Has anybody ever revealed a serious weakness in response to the "weaknesses" question, that was hidden throughout the rest of the interview? It seems that asking people "What are you most proud of" gets more or less the same info - what people struggle with most is usually their weakness - but keeps the tone positive.

In answer:
The question is not meant to be intimidating or place the candidate on the offensive. The question is to assess how honest or self-aware the candidate is of their weeknesses or improvement needs. When asked this most people try to be genuine and it is the type of question that can make you think. Whatever the answer you will gain insight into how the person thinks and that can only be helpful in the assessment process.

Firstly, it allows the interviewer to discover additional weaknesses either in the weakness itself or in the way the weakness is conveyed to the interviewer. So it is not just the weakness itself that is an issue, but how this is conveyed: ie - is it done professionally or reframed appropriately, etc.? This can help the interviews understand who socially aware and groomed a candidate is. Second, it also give the interviewer some insight into how the candidate thinks about his/her weaknesses. What do they consider a weakness? Why do they consider it a weakness?

The candidate who responds "none" is not being honest with the interviewer or themselves. The way most people will answer this question is to turn it into a positive, for example "I'm a perfectionist, I need to review all the details and ensure what I am presenting is the best". This is a way to put a positive twist (which most interviewers are aware of), a good interviewer will dig deeper to determine if this gets in the way of doing their job.