Professor L. Randall Wray on why a government with a sovereign non-convertible currency might choose to issue bonds. Bond sales are not a borrowing operation for the state. Logically, since the dollar is a liability (an IOU) of the government, it’s impossible for the government to borrow back dollars, just like it would be impossible for you to borrow back your own student loan debt, or for Pizza Hut to borrow back its own coupons. Rather, a bond sale is just a swap of one government-issued asset (cash) for another (bonds) which pays interest. It doesn’t change the amount of assets or liabilities out there, only the form.
A government that issues its own non-convertible currency does not need to sell bonds in order to spend. This is because it issues the currency every time it spends (and destroys the currency when it taxes). The main reason such a government might want to sell bonds is because of its effects on interest rates.
If the government is running a deficit, then it is creating more money than it destroys through taxes. This means that the banking system will have excess reserves, more than they need to settle inter-bank payments and meet reserve requirements. Normally, banks don’t want to hold excess reserves, they’d rather purchase some other higher-interest-earning asset.