Its kind of a BS question. But I think it holds water, at least a little. You went through a lot of school -- why? What career path were you trying to open that would not otherwise have been available, and how do you see yourself progressing in it? Truly not knowing is a sign that you haven't looked into the career and progression much. It means the hiring manager has less of a signal on if you'll be happy at the company in 1, 3, 5 years, because you can't tell them what you are looking for.

I was asked a similar question when I was applying for Medical School, and told my non-answer was BS. It frustrated me at the time. 10 years after quitting medicine, I can't say that person's viewpoint was wrong.