Why would anyone want to buy a bitcoin address?
For example to claim other coins.
For Transaction History or using for claim another coin. If i remember correctly, they buying address and signed message it to claim free CLAM coin in the past, so i think its work like that too now.
But in our case here, it has nothing to do with claims. This pivate key that has been used to sign that massage is one of the oldest, maybe the oldest that exist.