It tests the candidate's ability to work on a vaguely specified problem and also gives them insight on how the candidate feels about working on a lot of poorly specified projects.
If the candidate is not smart enough to get up and leave the interview after being asked such a terrible question, you don't want to hire them. The company's actual business model is to sell the resumes of people who are smart enough to reject working there to companies looking for good coders.
If the candidate is not smart enough to get up and leave the interview after being asked such a terrible question, you don't want to hire them. The company's actual business model is to sell the resumes of people who are smart enough to reject working there to companies looking for good coders.
Edit: confusing use of "their".