For the last few days now, I've seen some questions either closed or moved to SuperUser.
Why is that?
Because SuperUser is for questions that are from a general hardware or software perspective. Many people will mistakenly think that StackOverflow is going to help them with making their computer work, so we will migrate it to our other site.
Also, ServerFault exists for questions that are more networking and SysAdmin related.
Because the sites each have a different content and audience focus. See the FAQ for details.