Interestingly a lot of the wiseacres in the comments don't either.

There are some interesting questions asked...The one that is never answered properly is "Why does the government borrow money at interest ( leading to accumulating debt) when it could just print it without?"

The answer is that capitalism can't exist without feu. The interest is in effect a form of tribute: a submission to the idea that money should always be governed by private ownership...even though it's simply not.
Interest yeilding gilts and Treasuries are merely notes of submission to the private gods of Capital...there is no real objective explaination however hard you might look.