As explained in the OP, it's useful to have a direct link to the base page (that is, first page) of the thread--because while I'm in the thread, I'm typically not on the first page. If I need to copy that link, it's easier to grab it from the header than by having to mouse over to the much smaller link in the thread-page navbar.

But most obviously, this is how things used to be. Then that useful feature was removed for no apparent reason.