Lets just say if some people did not have money, they would be as useful as a wet tissue.
If I didn't have money, I could offer my skills to barter for other goods or services. This is what money fixes, it creates a field where everyone can play on.
The problem is when all your skills and what you have to offer gets automated by machines that are 100000% more efficient and cheaper than you.
This would make no sense on a larger scale. Machines exist so that humans could switch their activity to something more interesting/rewarding. If all men would one day become unemployed, then there would be no market for the goods machines produce. So it is a self-balancing system with a feedback loop...