Can somebody please explain to me what is the point of the overlay header in some websites (like OP's)? Do people really want to reduce the content's available

Can somebody please explain to me what is the point of the overlay header in some websites (like OP's)? Do people really want to reduce the content's available screen real state because of the header? I really don't see the point of it and it's one of the main reasons I love readability. Do people really want the header present at all times while you are reading the page's content? It's not inconvenient to just go to the top of the page in case you need a link from there but maybe I missed the usability tests proving otherwise. This is a genuine question btw, so if anyone could inform me I would really appreciate it.