> Why don't we delete these instead of closing them?

- To serve as an example of what is off-topic here. The rationale is that people who see off-topic questions closed remember them and don't ask any off-topic question. The same goes for "bad" questions (i.e. "not constructive" ones).
- For historical purposes (some people really *want* these to stay, which would lead to delete-undelete wars)
- As a stage prior to deleting (e.g. you can go through all the "Not a real question" questions and delete them sooner or later)
- etc.

> And when is is appropriate to delete a question?

When it is …

- absolutely off topic
- insulting, offensive
- spam
- not valid anymore
- of really bad quality
- etc.

The reasons vary here.


----------


See this blog entry for more details: 

> ## [The Stack Overflow Question Lifecycle][1]

> ### Why do you allow content to be deleted?

> Just like death is an unfortunate but normal part of life, I believe deletion is also an unfortunate but normal part of living websites.

> …

> ### Why would you delete a question? Isn’t closing it enough?
> 
> - Some questions are of such poor quality that they cannot be
> salvaged. They’re literally nonsense. Not every byte of data that is
> created in the world is infinite and sacred.
> - Some questions are so incredibly off topic that they add no value to
> a programming community.
> - The mental cost of processing these closed questions is not zero,
> particularly for users who are actively engaged and scanning questions
> to find things they can help answer.
> - If users see a lot of closed questions, they’ll note that we don’t
> enforce the guidelines, so why should they? Without any final
> resolution, asking questions that get closed becomes something we are
> implicitly encouraging — a broken windows problem. If this goes on for
> long enough, we’re no longer a community of programmers who ask and
> answer programming questions, we’re a community of random people
> discussing.. whatever. That’s toxic.
> - If enough of these closed questions are allowed to hang around, they
> become clutter that reduces the overall signal to noise ratio — which
> further reduces confidence in the system.


  [1]: http://blog.stackoverflow.com/2009/04/the-stack-overflow-question-lifecycle/