For the buyers, I always assume it has to do with taxes. "Proving" you owned Bitcoins a long time ago, and your current riches really came from capital gains and not crime. Or just bragging rights, but it seems like cheating if you bought the empty address.
That part about taxes is not very logical to me
It's not perfect, but I don't have a better explanation either.
unless the tax office is very inexperienced and naive to believe such a story. If I sign a message from a BTC address that is, say, from 2012 and show it as proof to my tax office, if someone there is intelligent, they will look for a history of transactions that will connect the sold BTC with that address
"Well, this is how the story went: I sold it for Monero back in 2014, and recently bought Bitcoin again. That guy who bought the address on Bitcointalk isn't me, I'm the one who sold it. After all, that's not illegal, right?".....
If this story works? Who knows. But it sounds better than: "Well, you're right, I'm a hardened criminal who belongs in jail".....