Two examples:
SE: You can create a question but not delete it (and there is a limited of time to delete a comment).
Quora: As far as I know you can't delete a question.
What are the advantages of this?
Two examples:
SE: You can create a question but not delete it (and there is a limited of time to delete a comment).
Quora: As far as I know you can't delete a question.
What are the advantages of this?
As @kontur said, I think it comes down to content strategy.
Is the user generated content valuable to people other than that user? With Q&A sites especially, the answer isn't only for the person who posted the question, but for all the other users who might come searching for answers to the same question. Limiting the ability to delete means protecting the user experience of these other users - making sure they can find answers too.
Also, with sites such as this the concept of "Your Questions" is not very central. In an email app, I have an inbox that I want to keep clear so I will delete or archive items. Same for a todo list etc. With a Q&A site or a social network, there is a constant new stream of content and the focus is not on a list of things you have to "complete", so the user is less likely to feel compelled to delete them.
Consistency is the main reason. When content has (arguably) relevancy beyond the users own scope, I.e. tweets are considered "news broadcasts", it would be confusing, if those items were getting deleted. In your SE example the website representative decides if your content is worth keeping available for all users.
Considering content strategy this also ensures that users take responsibility for the content they submit and thus it could be argued that it improves content quality.
In the case of twitter another consideration is that items are generally viewed as containing only temporary value for a short time. Unlike a article a tweet does not need revising; whatever "change" happens to it gets updated via a new tweet.