To express that you think so, but are not totally sure. Other example: You write/say a sentence which you think is correct, but you are not 100% sure, you might ask: "Das ist doch richtig, oder?" It is just to stress that you think it´s correct. It works just as well without the "doch".
I tried, "the woman is beautiful, don't you think?" because it is the first thing that came to mind that seemed to make sense of it. I would argue that it means the same thing and is a natural way to say it. However, I am not offended that Duo doesn't accept it, since there has to be some limit on colloquialisms; they can't accept every possibility