> Sometimes it's useful to be able to prove a file was created /before/ a given time.

Let me give a personal example: some time ago, someone deleted the public copies of some webpages we were using in our investigation and threatened to us that if we continued, they would simply claim that our copies of were forged later (after we had contacted them), and, they thought, make us look like fools & scuttle our claims (with some hints that they might also sue us for libel to boot). Fortunately, I had been timestamping all along and had timestamped the archived webpages in question; so I could prove that my copies are unaltered from well before they raised the question, and, further, at around the same time as they were independently captured in a different archive service. After we noted this to them, they had to back down on the threats and take us more seriously.