So if you do not use HTML5 tags, why declare your page as HTML5?
there are a couple of advantages for using HTML5, even if you do not require any entities from it. For example, the doctype and encoding are well defined. The code is also simpler, since it cleanly defines tags without closing tag. And it carries a version information - so the browser does not need to guess which html version you want to implement. Lastly, it is also a much conciser standard than all html before - with cleaner separations of concerns. HTML5 is a stable basis for website for the next decade, nothing before really is.