While this gets a bit technical, the operational purpose of such bond sales is to help the central bank hit its overnight interest rate target (called the fed funds rate in the US). Sales of treasury bonds reduce bank reserves and are used to remove excess reserves that would place downward pressure on overnight rates. Purchases of bonds (called an open market purchase) by the Fed add reserves to the banking system, prevent overnight rates from rising. Hence, the Fed and Treasury cooperate using bond sales/bond purchases to enable the Fed to keep the fed funds rate on target.
You don’t need to understand all of that to get the main point: sovereign governments don’t need to borrow their own currency in order to spend! They offer interest-paying treasury securities as an instrument on which banks, firms, households, and foreigners can earn interest. This is a policy choice, not a necessity. Government never needs to sell bonds before spending, and indeed cannot sell bonds unless it has first provided the currency and reserves that banks need to buy the bonds.
