understanding rel="attachment wp-att-###"

when I attach an image to a page or post and look at the code, I see rel=”attachment wp-att-###”. I used to delete it and replace it with rel=”nofollow”. I have never understood what the point was of having that relationship specified in wordpress. does it help sep or does it hinder it? or is it just a formality that is there that you could get rid of without any impact. any feedback will be appreciated. thanks.