The best example is when I bought my first generation G4 in 1999. It came with OS 8.6 installed, but OS 9 came out soon afterward. Apple provided a promotion where I clipped the proof of purchase, sent it and $20 US to them and I recieved OS 9 disks for my G4.
Normally, with OS updates, it works within three months. As an example. if Panther is released three months from today and you bought a PowerMac or whatever with Jaguar, you could send it the Prrof of Purchase along with a copy of your receipt and Apple will send you the update for a shipping fee. I've only sent in once for 10.2 after I purchased an iMac that came with 10.1.5.
That would be nice. No, those are your only true Proof of Purchase of the OS. So whenever there is a promo that says mail or whatever your Proof of Puchase of OS X, thats when you use it. They are not coupons in the regular sense. They are more like...UPCs that you cut off ceral boxes and send in to get the toy or whatever...(now THATS something I havn't done in a while)