According to MMT (Modern Monetary Theory) governments sell debt in the form of bonds in order to create a stable store of value for investors. They feel safer making risky investments if they know that some if their money is safe. Governments, particularly governments with fiat [1] currencies, are extremely safe borrowers. You can more or less guarantee that their bonds will be paid back. Because if a government wants to pay that bond back they are incapable of running out of money. So governments could buy their bonds back and forgive them but it would defeat their actual purpose. I read about MMT in The Deficit Myth. It's an interesting book and worth the read id you want to a different perspective on monetary theory.
Fiat in this case means the currency is not based on any asset. Common currency basing assets are gold, silver, petroleum, and cryptography.