elliemay - It isn't obligatory but it makes sense to someone asking a question , then getting an answer not knowing the reasoning. This happens loads of times when posters think they know and answer but don't know the parsing.
I suppose it goes down to something which I have said several times - give an early answer and hope to get a meaningless BA which I believe you delight in.
Many posters say that they don't feel they have completed the puzzle if they can't see where the answers are from.