There is a certain breed of photographer who works on the margins of society or under conditions that few of us would ever dream of approaching. Besides Arbus, Gary Winnogard and Weegee come to mind. But also in this category I would put war photographers such as James Natcheway.
I believe the reason these individuals were (are) able to produce such moving and provocative images is because they share an almost voyeuristic relationship with the subject matter. That is they have an obsession, even a need to view the subject matter. The camera is a tool that helps them indulge in the obsession. I am not saying that it is good or bad, just trying to suggest an explanation for how such bodies of work come about.
The ironic thing is about her work is that she transforms anyone who looks at her work into a voyeur right along with her. I still remember when I first became aware of her work. I thought some of the images were pretty disturbing or at least the product of a disturbed mind. But that did not keep me from going back again and again to look at the images in the same book.