To add to the list -- another user already mentioned funding foreign clandestine organizations. The modern day equivalents of Iran-Contra -- another theory is a government or organization that wants to undermine central banking, or global reserve currencies.
Governments used numbers stations over shortwave for years for remote communication. It's not too far-fetched to see the utility of Bitcoin for a government entity. The only problem is that you still need BTC->local currency exchange to fund clandestine field work, and I'd bet there are several currencies you can't get via BTC.
To add to the list -- another user already mentioned funding foreign clandestine organizations. The modern day equivalents of Iran-Contra -- another theory is a government or organization that wants to undermine central banking, or global reserve currencies.