@πάνταῥεῖ Two reasons: to get more close votes, and to create a log of links to these posts. FYI I'm not the first to do this: Journeyman Geek does so too, but uses a z instead of an s at the end.
(If you have <3k, you close flag instead of close vote, and those links are always visible in your flag history. Once you gain 10k rep, your "votes" tab shows links to deleted posts as well. Thus, when you have enough rep to close vote but not enough rep to see deleted posts, you lose access to those links.)
Top bar is ... probably not the term I want... Teams, for example, uses buttons that looks like this and that's part of the responsive design elements.
Search for an example ticket on Southwest Airlines, and go to the fare selection screen...you'll see a button "Unlock header" to prevent it from scrolling with the page.