Reposting because it was removed from another comm earlier for the political content.
... What? No they weren't
Anyways, it's not about coins, it's about taxes. Taxes can be collected in the form of currency or in bulk goods
If you don't have any coins or bulk storage, your community is not economically productive for whoever owns the territory. There's nothing to take
But infrastructure must me maintained anyways, an empire lives and dies on it's roads