Another use case is where you're doing a presentation using someone else's slides created months ago, with the original source document having gone missing. Literally, the only things you need to change are (1) the name of the presenter and (2) the date. So, typically, since you can't change the PDF at all, you simply perform a workaround by not showing the first slide (and you hope no one notices the date in the footer that is consistently wrong, slide after slide). Now, however, you're able to tweak the PDF! Change the name on the first slide and it's as if you wrote the whole presentation yourself, this morning, rather than having recycled it from some other presenter who created it three months ago and then lost the original OpenOffice document. (Personally, I can't imagine this scenario happening to me, of course. It's purely hypothetical and it was a strain to even come up with it, being as it is so far removed from my own experience!)
Plus, there's also a concept of hybrid PDFs: "Combine the viewing and printing portability of a PDF with the editing capabilities of OpenDocument Format. 'Have your cake and eat it too,' promises ODF embedded in PDF."
Clayton Cornell, from the OpenOffice team, who I met this evening in Prague, told me about all this and so I had to download OpenOffice and get all this working right away. And it was really easy. Just download and install it and then add the plugin via the Extension Manager under the Tools menu. Seriously, this is cool.