I don't really understand the massive mutual debt of countries either and I don't keep debt in my personal life.
I can see from a startup company perspective there are times when it's to everyone's benefit that you borrow money, in order to increase the speed of growth. For example if your product goes viral and suddenly there's a much greater demand for it than you can supply with your current equipment.
So I guess that would be to monetize a factor (the spread of information) that was outside of everybody's control.
But the majority of borrowing doesn't fit that model. It's either on the intangible promise of growth, which is often made on false assumptions and never comes true, or it's to compensate for an unexpected disaster. Or I guess, like in the case of the USA government lately, it's out of fear.