Purpose and reason

A: What did you do that for?
    B: I thought you wanted me to.
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    A: What do you want me for?
    B: Can you give me a ride downtown?
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    A: What brought you here?
    B: I just stopped in to say good-bye.
    A: Going back to Japan?
    B: Yeah. I have a flight tomorrow.
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    A: What do you go to college for?
    B: For social reasons.
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    A: What have you come back here for?
    B: I forgot to tell you something important.
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    A: What are you here for?
    B: I came to see Jane.
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    A: What did you come to L.A. for?
    B: To study English.
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    A: What is your goal this year?
    B: I want to sell three new Fords each week.