Updating question - reason for update

Sometimes, when updating / editing a question, I see a box asking me to put in a comment about why I'm doing this; other times, I don't. Any ideas about why / what's going on? Sorry, the situation / discussion is a bit vague. I'll update if I figure out anything else relevant... --edit-- I think I may be mistaken. I don't get the update reason while I'm editing a question, but do while editing an answer... Should the reason-for-update box be there in both cases? And should it be mandatory to fill it out?