It's a good thing!
The site's goal is to build a usable, accessible knowledge base that will be of use to future users.
Most of the time you'll find that users who 'self answer' have been unable to find the answer to their question and, through some research, have determined the solution to their own problem. If you want to share the answer that you've found then there has to be a question on the site. If there isn't an existing one, you'll need to create one which is often how self answers turn up.
Check out this canonical meta post that explains it more eloquently than I can.
To address your second point about length answers:
It is about quality not quantity. Just because the answer is long doesn't mean that it is good. If you think you can provide a better answer then I would encourage you to provide it. A clear, succinct answer can sometimes be much better than an essay!
Remember that if you feel the question or answer are not useful, you can downvote the post. Voting is important and a key tenent of our site's structure.
If both the questions and answer are consistently good, there is nothing to worry about. It's a great way to gain rep too.
- If the questions or answers are falling below your personal standard, vote accordingly.
- Consider providing your own answer to questions, even when they already have an answer by the OP, if you feel you could write a better one.
- If the questions or answers are falling below the standard we expect on the site, flag accordingly.