In addition, it tells folks how much available content. And you can eyeball an action like “go to approximately 80% down the page.”
Seriously, that's another thing I hate about infiniscroll: if I come to a giant list where I know I've looked at the top 10% of, it makes it impossible to jump down approximately 10%, or otherwise to a region I know I haven't explored.
Edit: And, of course, if you do want to go n% down, there's no choice but to page down, wait, page down, wait, page down ...
However, that one I blame the browsers for. I distinctly remember the back button working differently, where it would reliably take you to the exact state of the previous page, just as if you had opened the current link a new tab and closed it, but it changed at some point, and most sites do something that destroys the state when you back into them as well.