The octothorpe/number-sign/hashmark has a special significance in an URL, it normally identifies the name of a section of a document. The precise term is that the text following the hash is the anchor portion of an URL. If you use Wikipedia, you will see that most pages have a table of contents and you can jump to sections within the document with an anchor, such as:
You will see the page refresh from the server. Now type:
window.location = 'http://minimal-github.gilesb.com/raganwald#and_this';
p.s. There is a fourth benefit to this technique: Loading page content through AJAX and then injecting it into the current DOM can be much faster than loading a new page. In addition to the speed increase, further tricks like loading certain portions in the background can be performed under the programmer's control.
"However an application without this optimization is still crawlable if the web crawler wishes to index it." Not really. The hash doesn't get sent to the server.
just for information: self.document.location.hash provides the value of this hash
@imaginonic: I'm late, but as perfectly crafted as it is, 90% of it doesn't touch on the #! aspect of my question at all. That's why he said it's redundant. The number of upvotes here is likely due to the high traffic when my question made it to Hacker News coupled with the sheer length alone of this answer.