What is the purpose of closing a question? [duplicate]

Question

Having been a member of Stack Overflow for a year, I have participated in reviewing close votes and flagging questions if I found any duplicates. However, I don't understand the purpose of closing a question. As per myself, the main purpose is keeping the site clean and make a better place.

1. I still see closed questions on the site that are a year old. When will these questions be deleted?
2. Also, when a question/answer does get deleted, it is still visible to 10k users, even if it is deleted by the owner.

Every day I participate in reviewing close votes, but the question arises why we are preserving these questions and answers even after they are closed.

Answer 1

The purpose of closing a question is to prevent answers from being posted while the problems with the question are fixed (a process I call "rehabilitation"). Without closing, there's no motivation for a user to improve their question.

Not all questions get fixed, for a number of reasons. Similarly, not all questions get deleted, as some questions (despite their closed status) still confer some value to the community.

Questions never get hard-deleted, unless you offend someone at SE corporate (the "Boat Programming" question is the most notable example). In theory, any deleted question can be rehabilitated by a high-rep user. In practice, there are many reasons not to hard delete; historical preservation and the ability to do statistical analysis being two examples.

Answer 2

Answering the bullet points:

1. Questions that have some degree of quality (i.e. at least 2 upvotes on an answer) aren't deleted. It assumes that the answer is of sufficient quality and, therefore, the question "could" be eventually rescued.

2. Having in mind that the purpose of SE is to gather all the world's (insert subject here) knowledge, it doesn't make sense to have a hard-delete, as any question in itself has a slight bit of knowledge. They can be rescued, repaired, and made worthwhile... Still, the fact persists most of those are not all that good, are unrescueable, and as such can only be seen by 10k+ users.