In any position there is a learning curve. You slowly learn new things and become more effective in your role. Sometimes you have these small discoveries that you wish you knew from the start. In every role they are different. This is a great question if the coworker you are interviewing is in a similar role as the one you are interviewing for.
Evaluate Job Description: You can use this question as a means to understand what the coworker thinks should have been added to the job description. There are things that someone conducting the job may have struggled with that management does not understand. Sometimes, if you are lucky the answers could help you better assess the inner workings of the job.
Learn From Experience: This is a great opportunity to learn from the experience of others. The answers will give you insights into what the coworker think could have been beneficial. Perhaps there are certain things you should know about the structure that would make your life easier? An answer could also offer insights into the mentality of the staff or the corporate culture.
Make the Interviewer Think: The interviewer could be caught off guard by this question. They may have to think before they answer. Perhaps they will remember you for the ingenuity of the question?
Use Knowledge to Better Position Yourself: The interviewer will tell you what he or she believes could have helped them. Listen to the answer, try to absorb it. Is there something you can learn from this and take into your following interviews? Is there anything you can add in light of this that will strengthen your application with this interviewer? Think about how you address this. You could for example say that I had a similar experience when I joined my previous company and then share something that you learned only after a while at that company. Perhaps you have learned the same thing as them with your previous employer? Then you can add to their story and show that you have things in common. Remember to be friendly and nice in your report.
Assuming Others Know What They Are Doing: This may sound harsh, but it is reality. Not all people know exactly what they are doing in their role, so their advice may not be useful.
We Struggle with Different Things: What the other person struggles with might be something that you would have no problems tackling. We are all different and evaluating the company based only on this question is probably a bit too harsh. You could gain useful insight with this job interview question, but do not assume all people struggle with the same things.
All Interview Questions for the Co-Worker