I assume you are familiar with the concept of an interrogative sentence, and you specifically are asking about questions, not interrogative sentences, which are characterized by their form. However, I grant that people often disregard the difference between form and function, to their detriment, I would say.
Questions, then, are characterized by their function: they are requests for specific knowledge. We can take it for granted that having knowledge is a good thing, and ignorance is a bad thing (though perhaps that fact needs separate discussion). Questions are therefore good, because they are solicitations of something good. Of course, questions are not an absolute good, that is, there are contexts where asking or answering a certain question is a distraction. Since all question-asking ultimately makes a claim on someone else's attention, there are contexts where a given question would be appropriate, and ones where it would not be (that is, the "cost" of asking the question is too high, compared to the benefit of expanding knowledge).
Returning to the matter of form vs. function, an interrogative sentence like "What the hell do you think you are doing?" cannot function as a request for knowledge. It is plainly a demand that someone justify their actions, which is something totally different. "What is this" may function as a request for knowledge, but it may also function as demand for justification, depending on whether the item is an obscure plant vs. the speaker's screw-driver (on the assumption that the speaker does know what the thing is). It can be very difficult to determine the function of an interrogative sentence.