To complement Alex's answer, I'd add:
One reason people (including me) sometimes ask for the addition of explicit motivation is that the question is that the question is of the apparently-arbitrary form "What does Judaism think of X?" where it's not inherently obvious that Judaism is likely to say anything about X. Adding motivation makes it clear that the question is not a throwaway question and is worth answering.
Another situation that can cause me to ask for such an edit is when it's not clear at first how the question is about Judaism. For example, a question about techniques to achieve some practical goal may not be obviously about Judaism, and may therefore be deemed off-topic, but may become clearly about Judaism with the addition of explicitly Judaism-borne motivation.
In the case posed here, assuming the curiosity-based question is clearly about Judaism, it seems unlikely that it would get closed, but if it really strikes people as having no clear value, it's possible people would choose to vote it down as "not useful." Of course, everyone's votes are their own to cast.
The bottom line is that, regardless of any rules articulated on Meta, if you want people to invest time in reading and trying to answer your question, to choose to vote favorably on it to give it more exposure, and to find it on Google, it's in your best interest to make it as compelling as you can to as broad a base as you can. Long-tail questions are fine, and we've got plenty of room for them, but we can't guarantee they'll get read or answered. If you get a comment asking "why do you want to know," I recommend that you take that as a helpful indication of one thing you can do to increase the question's chances of accruing favorable attention.