There may be a couple reasons they want to see them. It may be medical. If you are cutting really deep or a lot it could pose a health risk. It may also be she wants you to be more comfortable and be able to talk about it. To help you see more clearly what you are doing.

My therapist never wanted to see mine. She knew I self injured, but I always did it on a concealed part of my body. I think she was okay with just talking about it. I don't think she can force you to show her, unless she thinks you are a danger to yourself.