Fundamentally, why are there fiscal rules in the euro zone?

In theory, the euro-zone countries are subject to strict fiscal rules concerning structural fiscal deficits and public debt ratios. These fiscal rules are now the subject of debate, as the very low real interest rates may enable higher public debt ratios and fiscal deficits. This raises questions about the underlying reasons for the fiscal rules in the euro zone: The idea that high public debts could be monetised, leading to inflation. But this has visibly not been the case in the recent period; The idea that a high fiscal deficit in one euro-zone country drives up its interest rate, reducing investment in that country. But this is then a problem for that country to be addressed by its government, and not a problem for the euro zone; The idea that a high fiscal deficit in one country generates a negative externality on the other countries by driving up the common interest rate in the euro zone. But in practice, a high fiscal deficit in one country drives up interest rate s in that country alone (there is “market discipline ”), possibly because the euro zone has abundant savings that prevent the interest rate of the euro zone as a whole from rising; and possibly also because investors discriminate between the countries. Fiscal deficits therefore do not generate a negative externalit y . Altogether, the underlying justification for the fiscal rules in the euro zone is not clear.