Q. How is it possible that Rashi sometimes says he does not know the answer to a Biblical question (e.g. on Gen. 28:5)? What use is a commentator who does not know the answers?
A. The Talmud tells a sage to be honest and humble enough to say, “I do not know”, and not to pretend or to tell an untruth (B’rachot 4a). Even the greatest scholar can be baffled by a problem. Why not say so, and add, “But I’m going to try and find out!” Without this determination to continue to look at the problem, the humility that produced the “I do not know” can, God forbid, turn into an egotistical boast, “Look how humble I am!”