This might seem like a joke, but probably organized crime. Like the Cartel or the Mob.
Say they have to move money from US to Mexico, but millions in cash is too obvious to move around. They can approach someone in the USA to sell them BTC for cash, then the Cartel moves the paper wallet to Mexico. After that they just get someone in Mexico to buy the BTC for cash.
If you are the one in the US trying to sell BTC, you get to do a block trade, dump BTC and get all your money at once, provided they don't kill you right after and just keep the money plus the BTC.
I am sure this is how its done with silver and gold. Lots of organized crime types will approach bullion dealers and try to buy gold/silver WAAAAY over spot, in exchange for them looking the other way.
What does this have to do with my question?