Outside the context of HN, many sites put their name after the name of the article in the article's TITLE element (usually with a separator like a pipe or chevron) so that if the user bookmarks the article the site name is included in the text of the bookmark. That has value, though less than it used to now that browsers display the site's favicon with the bookmark.
In the context of submitting to HN, though, you're right, it makes no sense. My guess is that people just grab the content of the TITLE element and don't bother doing the extra work to strip the site name out.
In the context of submitting to HN, though, you're right, it makes no sense. My guess is that people just grab the content of the TITLE element and don't bother doing the extra work to strip the site name out.