It's also how someone might respond to any question if unexpected, or if they are trying to get through a meeting with the public that they have to do as part of their job but it is the part of the job they are less than comfortable with - put on an energetic happy-go-lucky public persona and plow through that with some Great Questions! and Wow, I'm glad you asked that! End up seeming a bit manic but at least they get through it.

It's also how people will answer bad or a bit silly questions, and then try to turn it into a great question by rooting around in it and pulling something great out so as not to hurt people's feelings. Because of the whole there are no stupid questions thing. (just listing other reasons why people answer with Great Question, not insinuating anything here)

It is also how someone might be expected to reply to a Great Question.