Many projects take a long time to become profitable. The workers would starve until then.

Because of that, they need to borrow wealth to be able to keep working on the project until it becomes profitable.

The concept of lending wealth is therefore necessary.

Noone is gonna accept the risk of lending out wealth unless they expect some gain, some profit.

Therefore the workers promise to not only repay the lent wealth but to also pay on top of that, perhaps a share of the project's profit.

Hence the concept of profitable investment is necessary.