More than once I have been in a situation when a proper response to a comment just doesn't fit within the 600 character limit & the better approach is to edit the answer.
At times like these, being able to see the comment thread would be a real benefit. When a comment addresses multiple issues, I typically find that I address part of the answer, save, compare the comment to the current state of my answer, then repeat the process until I'm done.
I can open the edit page in a new tab, but frankly I usually never think about this until I've been editing & I realize I want to reread the comment. Sometimes this happens on questions too, but less often for me & I see that question has already been asked.