I noticed that by default the there is no category in K2 after fresh installation. But Joomla core com_content has a default category named "Uncategorised". This "Uncategorised" category can be used to hold articles that not suitable for any category, and usually aimed to be some "independent" articles, such as "about us" or "special announcement".
Since K2 does not allow to create articles in "root" category, I think an "Uncategorised" category is necessary to hold this kind of articles. Putting "indenpendent" articles in "Uncategorised" category is a good idea because when SEF enabled, SEF component (like JoomSEF) will skip its category name when generating SEF URLs, so we can get:
http://www.abc.com/aboutus.html
instead of:
http://www.abc.com/uncategorised/aboutus.html
If you agree this idea, please add a new category named "Uncategorised" by default and lock it to avoid modifying its name or deleting it.
Thank you.