It is based on a theory. Essentially, the WACC (weighted average cost of capital).

The idea is that all “Capital” whether from a stock holder or bond holder demands some rate of return. Bonds interest coupons are explicit, stocks return whatever profits remain.

When you think about it that way, adding debt actually allows for more growth/spending because the overall (weighed) rate of borrowing actually decreased before it increases when interest rates are low (and the tax shield helps since some interest in debt is deductible - usually about a third of it).

It’s leverage. It does increase risk. Just imagine if you used a credit card to buy a tool and used this tool to expand your business. Same concept. You’d better be sure that spending will pay for itself.

> Can someone explain this puzzling aspect as to why having a mountain of cash ($200B in cash and short term investments) is so bad, but having massive liabilities is considered to be a good idea?

There's an opportunity cost in having that cash on the balance sheet, since it's cash that isn't working for a return. If the expected return on keeping that cash is less than the cost of capital, you're effectively burning that cash by keeping it in the company bank account.

This means that unless the company has a good reason to keep it (ie: they want to use it soon for a big investment) they should be returning that cash to its owners, so they can reinvest it elsewhere.

Whether you should be funding your business with debt or equity, depends on your cost of debt and your cost of equity, which in turn depends on your business model.

For a company like Apple, with huge revenue and profit generating capacity without needing substantial capital assets, a good credit rating in a zero interest rate environment, the cost of debt is going to be fairly low and as such funding the business with debt is more attractive than equity.