Q:

What is a "useful" question?

When one hovers the cursor over the upvote arrow for an answer, the legend says "This answer is useful". This is a somewhat subjective statement, but for me clear enough: a useful answer is readable and apposite to the question; it settles the question to a fair extent at least; and it goes beyond, or does better than, or at least provides a reasonable alternative to, previous answers. No problem.
Where I have a problem is with the corresponding message for a question: "This question [...] is useful [...]". I am puzzled by what a "useful" question might be. The idea of a "useful question" would no more occur to me than "useful planet" or "useful symphony". I'm not saying that the concept is absurd; it's just that I don't know how to interpret it, and I wonder whether there is general agreement by others as to what it means.

A:

This site is a collection of a certain type of questions (with their respective answers). Given this context, a question is useful if it is a useful addition to this collection of questions.  
What this means specifically will depend on ones conception of the scope and the goals of the site; but irrespective of those details I believe the definition given above makes sense.   

