The mithril-mail and Sting are signs of the free people’s complete victory over Mordor, proven by their ability to reclaim some of what was lost without agreeing to Sauron’s terms. Frodo and Sam are treated like royalty, demonstrating Aragorn’s respect for bravery and sacrifice over any official rank or status, and proving to them that their treacherous quest was vital to Middle-earth’s freedom and survival. But Pippin, like Sam, feels that a journey isn’t complete without being properly remembered, so he thinks it’s vitally important for Frodo to record every detail.