Well, I guess if you had money that was somehow locked away permanently, with no way to access it ever again, there'd be no way to definitively prove the money was yours. Even if you knew deep down that you earned or owned that money at some point, with no private keys as the rightful owner you'd have no evidence to back up your claim over it.

So, in that kind of situation, how could you conclusively prove the money doesn't actually belong to someone else instead? If you can't verify your own ownership of it, the door is open for another party to potentially step in and argue it's their money that got locked away. And, if it is someone else's, then it is not lost.