This question already has an answer here:
I have seen many questions by users on 'SO' who post question without providing code and/or less description and then right after posting question post answer. What is the reason of the asking question and posting answer him/herself right after posting question?? The only reason what I think is users who do this just want to show their knowledge. But I think if people want to share their knowledge then they should better write blogs and/or tutorial instead of posting questions and answers on SO.
I think a user should not given authority to post answer to his/her own question for like 30 mins to few hours after posting his/her question. Probably, other users could also give better answers because everyone thinks differently.