Without the third guy standing there with a club it's not clear if or why they would demand dollars.
Well, maybe sales taxes when he spends his ill-gotten gains, but that's indirect and if he's effective at intimidation with his club, he arguably doesn't need the money in the first place as he can acquire goods through other means.
The more compelling argument is that we pay taxes primarily to enforce peaceful law and order, primarily through control of force - at home through the police and abroad through the army - so that our stuff isn't just taken by "someone with a club" and making it worth being part of a larger society.
You could argue that government isn't much different to a mafia, except that modern taxation is generally more transparent, fairer, impartial, and perhaps more considerate to the people with the least money. Also, they generally don't use force to enforce taxation, only the threat of a prison sentence.
It can be a useful metaphor to visualize the economy as a web of interconnected loops of cashflows with "the government" on one side and the rest of the economy the other. By raising taxes it pulls some of its inbound ends of the loops tighter and by issuing subsidies/grants/credits and loosens up the slack on its outbound ends. It doesn't actually matter how much money "the government" has, because it's the final arbiter of how much money is actually in circulation, it only exists because it issues it. In theory it can play this game to achieve good social outcomes. In practice it's more difficult to predict far-reaching consequences of some economic action.
You open say, a hard rock mine somewhere in the US, you have to get mining permits to do so which are paid in...local currency.
If you don't pay your taxes in your nations currency you go to prison.