This means that the currency in use in a country only has value because of the legal decree of the government of that jurisdiction declaring it to be its legal tender and, more practically, because its value is backed by the ability of that country to raise future taxation revenues. This capacity is partnered by the ability of the government to command its central bank to make payment to anyone to whom funds might be owing using newly created money, if necessary. In this situation, there is no reason for a government to borrow from financial markets.
But, but, government borrowing is just offering investors a safe form of savings. Which government should increase the availability of by borrowing more, surely?