Every year we have a budget deficit because the US government always spends more than it takes in. The sum of all those annual debts is the National Debt. I find no evidence that the National Debt has gone down in modern times, say since the 1929 stock market crash. Selling bonds is just a mechanism to generate money to pay current bills. The man who built the bridge gets paid on time, as per contract. When the money runs out, some must be borrowed, which is where the securities come in. That pushes the debt forward and allows the politician to point a a shiny new bridge and say "look what I did for you".