It comes from economic specialization and the benefit of trade.
Whenever you draw a supply and demand graph, traditionally as a big 'X', draw a horizontal line from the left of the intersection of supply and demand. The area above that line and under the demand curve is the consumer benefit of trade. The area below that line and above the supply curve is the producer benefit of trade. That is the surplus produced just by the act of going to the market instead of trying to do everything yourself.
Ford's employees build more cars than they could possibly use themselves. Ford's customers have to bring something to trade. The existence of money lubricates the trade process. One customer may, after following the cash flows, do plumbing repairs in all the houses of all the employees, and in the Ford-owned buildings. It is easier for that plumber to fix a thousand toilets than to build one car. It is easier for the factory employees to build one car than to fix ten toilets. So if the plumber trades 100 toilet repairs for one car, everybody wins. The plumber, wanting the car, gets one with 10% the effort, and the workers get 10x more toilet repairs with the same effort. Ford himself decides the proportion of toilet repairs that go to him, and how many go to each worker.
The money price of goods and services continuously rebalance to reflect the desires of customers and the capabilities of producers.
In modern times, the owners of robotic capital are essentially allocating themselves--by means of adjusting money payments--tens of thousands of toilet repairs per year, and maybe half of one repair per year to each of their human employees. The robots, of course, don't poop. This just doesn't work. The owner can't possibly break that many toilets, and the employees are forced to repair their own toilets in addition to their regular jobs whenever they break more than once every two years, which reduces the amount of surplus produced by trade between specialists. You have to pay people enough for what they do or make to pay for the things they want, or you can't support economic specialization.
Ford paid his employees more because all of his employees were on average more specialized than the rest of the economy. Assembly-line auto production could not exist without specialization.
It isn't like swishing water around a bucket. It's like ten people adding one bucket of vegetables each to a pot and getting twenty buckets of soup out of it. The whole is more than the sum of its parts. Debt is more like a time machine for making trades happen that could not take place otherwise, as though the vegetables in the soup are all fresh when they go in the pot, but are harvested in different seasons.