The primary, overarching purpose of doctoral programs is to produce professors. Graduate school has a cult-like effect on what you think you should want as a graduate student. I have seen it turn people who thought they wanted to go work for the Defense Department turn into quivering blobs of jelly because they failed, five years later, to get a visiting position at Oklahoma State. It doesn’t matter what you think you want to do before you enter — there will be at least a part of you that drinks the Kool-Aid while earning the PhD.
Lest one think that I’m exaggerating, click on this Slate story about adjunct professors on public assistance. On the one hand, it’s obviously heartbreaking. But on the other hand, adjunct professors are people with PhD’s — skills and training that could be put to use for things other than teaching college students. There are options in secondary education, university administration, or non-academic career routes. But many of these people persist as adjuncts because they have been trained to believe that this is the only thing they should do with a doctorate.