Debt Mechanics

I've preordered and watched the gameplay videos.  One thing that puzzles me is how debt works.  I get that its 5x as bad vs. cash for your stock price.  But as I watch Soren J. play, I can't figure out the pattern.  Why have any cash on hand if you have debt.  Are there certain things that require cash on hand, and won't let you go into debt to purchase?  Why not let someone just go into debt then to purchase?  It doesn't look very intuitive compared to how real life works, so I'm looking for the rules.