Not every time that is required. But sometimes it is required to put additional description to the questions so that the question has more clarity.
Generally this depends upon the types of questions. If a question is getting too long, instead of putting it in the headline of the questions, it is better to put the further details of the question in the description box.
If the question is one type of debate based question and the answer could be multi-dimensional, then also the person who is asking the question, in addition to asking the question, his/her individual view point to the subject he generally puts in the description box.
You must have noticed that the description is optional, that means one is not bound to add additional description for every question he is asking, rather it depends upon the type of questions & varies from case to case and question to question. The analysis type, debatable type & quantitative analysis type questions may require the asker to put additional details in the description box.
There are many reasons that people would do this. One is that they are have a legitimate question and they want the world to see their general thinking on the question.
Such a post might be: "Who are you voting for?" and the post says "I am thinking about voting for X because of Y." The post is still a legitimate question. The answer just provides more insight in the author's thinking.
The mains reason that I write a post with a question followed by my answer is that I encountered the question outside of SteemIt and wanted to write a post about my answer. For example, I might listen to a political debate and what to show what I believe i the best answer.
Many people like to use Socratic dialog as it is a good way to present multiple perspectives in a single document.
Marketers like to use the FAQ structure.
"Frequently Asked Question" pages are often funny as the marketers develop both the question and the answer: [Here is the SteemIt FAQ](https://steemit.com/faq.html)
The final reason for using a question in a title is that the structure offers a tidy piece of click bait.
On a seeing a question, many people become interested in seeing the answer.
Well it depends on the question,there are some people that have an answer to the question which they are about to ask but their answer is not convincing enough to them so they are willing to hear the opinion of others so that they can be more enlightened about the subject which they are asking opinion about...
But also some people do that as a way of spamming on this platform in other to earn upvotes in whatever possible way they can...some people do it to spam while some people do it for a good genuine reasons...
They are trying to get the personal views of thosethat will be answering such questions. They try to let you know their view in regard to such question and they want to see if those answering such question would share a contrary view or agree with their views. Such type of questions are usually opinion based. The questioner tries to get the opinion of others by sharing a general opinion to such questions
Most people do that because they are in doubt of the answer they have already for the question and will want others to share in what their thoughts are.
Others do so just so that the person who will answer their question will share the same views with them and answer it from their own perspective.