Let’s say I have $1m in cash and I want to buy a house worth $1m. I could either buy the house with my cash, or take out a loan to buy the house and invest my cash into another asset.
Almost any investment will produce a greater yield than the interest cost of the mortgage because the mortgage is secured debt. A mortgage might cost 3% while stocks can return 7-10% or more.
In this scenario, taking on the debt even though you don’t need to is financially advantageous most of the time. In this sense debt is a tool.
Debt is also a tool that lets you afford a house that you can’t buy with cash on hand but where your income is easily enough for principal + interest. What’s better: a) saving for 30 years after you start working and buying a house in cash (maybe paying higher rent that whole time?) or b) taking on a mortgage, getting your house now (as soon as it’s responsible), and paying it off over 30 years. Most people will choose the latter.
Thus debt is a tool that also allows you to buy things far sooner (in life or business) than you could afford to with cash.
There are other things that can be done with debt like borrowing against existing assets (to avoid having to liquidate the entire asset just to get a little cash) or borrowing against expected revenue streams and so on. Most of these tools permit greater economic growth than is possible without.