Remember how Satoshi Nakamoto included the Times of London's 3rd January 2009 headline "Chancellor on brink of second bailout for banks" ? Why, what was he trying to prove ?
In a "longest chain is the real chain " rule setting, it protected those who joined the network against a possible "premined" copy of the Bitcoin blockchain : Satoshi (or anyone else) could not have secretly started and premined another copy of Bitcoin BEFORE January 3rd, 2009 (with the intention of mischievously replacing the common one with his own, LONGER private chain) because then the "rogue chain", started, say in 2008, could not have guessed what the Times of London's January 3rd 2009 headline was going to be. Its genesis block could therefore not have been identical with the real Bitcoin one.
Hence a public blockchain can serve as a time stamping device. The legal value in court is as yet unclear but in any case it's better than nothing.
So here is a practical use of this feature (thanks to ecency which stores the picture).
Local regulations require that new construction projects be publicly announced by displaying such a "certificate " (see picture below) on site.
Publishing this post today, it is a strong indication that this certificate was displayed on 12 March 2022 (or earlier) but not later.
It might even be worth testing the value of that proof in a court of law, to establish a precedent.
