Why do people ask questions at public events?

Ian Leslie, a loyal MR reader, asks a perceptive question:

Does anybody have a theory about the length of questions during the Q&A sessions that follow lectures/talks? Is there a relationship between length of question and age, gender, status, place in queue? Why do some people make rambling statements disguised as "questions"? How can moderators avoid such abuse of the process (pleas to keep questions short don't seem to have any effect)?

I see a few uses for public questions:

1. The "make a public statement and show them" motive.

2. The "somehow feel a need to void" motive.

3. The "signal intelligence" motive.

The "really want to know" motive is not absent altogether but I doubt if it is primary.

Anecdotally, I have found that men wearing suspenders are most likely to ask longish, rambling questions.

I am not sure moderators wish to avoid "abuse" of the question and answer process.  Perhaps the process is part of what draws people to the talk.

It matters a great deal if people have to write out questions in advance, or during the talk, and a moderator then reads out the question.  That mechanism improves question quality and cuts down on the first three motives cited.  Yet it is rarely used.  In part we wish to experience the contrast between the speaker and the erratic questioners and the resulting drama. 

My favorite method for giving "talks" is to offer no formal material but to respond to pre-written questions, which are presented and read off as the "talk" proceeds.