When a reviewer has a question, he or she posts a discussion thread.  The problem is that no one on the review team is alerted to the fact that there is a new discussion thread.  Likewise, if someone answers the inquiry by posting a comment to the thread, the original poster is not alerted to the answer.  If the discussion thread sent alerts, requests could move more quickly through the review process.  The alert I imagine is an email to the reviewers including a link to the portal discussion.