How does creating debt help grow the economy?

To help build businesses.

https://www.youtube.com/watch?v=YaxIPPMR3fI

According to the Keynesian model, during the deflationary gap, government spending will work as stabilization and growth of the economy. It has to be done by creating the budget deficit which is financed by borrowing. When the economy has picked up, the tax revenue will increase, an the deficit will turn surplus. To build infrastructure and Mega-project, it should be financed by foreign debts. It is the gain of technology and transparency management involved.

This is the Keynesian illusion used by demagogue governments.

It does not work, Roosevelt's "new deal" just delayed the recovery