I was going to give you the benefit of the doubt in believing you may not have understood the benefit of a document classification feature if you've never worked in an office with a need for such a feature but then I went to the website and noticed the BIG ASS GRAPHIC (https://i0.wp.com/blog.documentfoundation.org/wp-content/uploads/2016/08/classification.png?ssl=1) detailing the meaning of each classification which explained that the banner isn't for people who aren't a part of the organization.

If you don't care that it's confidential a banner wont stop you, if you do care, you'll take great care that this document doesn't end up in the wrong hands. (Make of that what you will) Its for businesses who don't have the luxury of having an air gapped corporate network to trust confidential information.