And it allows governments to play their proper role in stabilising the economy, which is essential in the 1/9  BIS central bankers' speeches  institutional design of our monetary union. So if sovereign debt is too risky, it will place the full burden on other actors to provide safe assets for the financial sector and safe liabilities for governments.
And because unsustainable fiscal policy can ultimately create pressure on the central bank to monetise debt, what is known by economists as "fiscal dominance", even in a fiat money system impaired creditworthiness of governments ultimately creates the risk of an eroding trust in the ultimate safe asset: central bank money.