One Law.SE, the site on which I am most active, it is fairly common to post a question and a suggested answer together, so that they post at the same instant. I have done this quite a few times, and had both question and answer upvoted. It is a way to share knowledge via an SE site. On ELL I think that would be less common, since the typical learner is asking but not answering questions here. It could be used by a learner who was confused on a point, and then has found what seems to be a good answer.
It could also be done by a poster who notices a side issue that has come up in a question, or more particularly in several different questions, and wished to give a response to such an issue. Such a poster might put the question in the assumed voice of a hypothetical learner.
But if a poster has an answer to his or her own question ready, I see no reason to delay posting it.