When you sincerely don't know the answer to a question or when you want to be sarcastic, by insinuating that the other persons knows more about the subject than you do..

When you know the person asking the question is supposed to know the answer better than you. Example: Your teacher: So do you think you have what it takes to learn English? You: I don't know. You've taught me for awhile now. You tell me! :-)

It is good to know: https://www.englishclub.com/ref/esl/Conversational_Phrases/N/now_you_tell_me__4306.htm

oh, this is easy, u can use it to response to someone when he/she knows the answer better than u. e.g.: a wife cheated on her husband, and one day when she kissed and huged the other man, a friend of the husband saw this, and told the husband, of course the husband got very upset and was ready to confront his wife. When d wife got home as usual, she asked the husband: how is your day? the husband answered: oh, I don't know, you tell me!! Got it?