The FAQ helps you a bit under the section "Why are some questions or answers removed?" stating that:
Questions that are extremely off topic, or of very low quality, may be removed at the discretion of the community and moderators.
Over time, closed questions that are not useful as signpoints to other questions may also be removed, as well as questions which have no significant activity over a very long period after being asked. For additional guidance, see How to Ask.
Every site has this section, I'm taking the MSO FAQ as an example.
So, apart from duplicates — which are usually those signpoints1 the quote above refers to — other questions are deleted after a certain amount of time.
In summary, the most common types are:
- Spam (deleted on sight);
- Extremely off-topic questions (closed/deleted/migrated soon after posting);
- Closed questions that don't serve any real purpose for the site (after some more time).
1: Duplicates are useful even when closed because if a new user finds the duplicate, instead of the original, this will prevent another "dupe's dupe", pointing that same user to the original question.