If my workers produce commodities more efficiently than the social average, my profit margins will increase, since my labor cost for each commodity will be less than that of the average producer. This is why the drive to decrease labor costs — to devote as little resources as possible to the well-being of workers — is intrinsic to any capitalist enterprise.
For the surplus value of our labor to be converted into profit, however, we must not only produce but also consume commodities, not only sell our labor-power but also buy the products of labor.
Only the employment of living beings who earn a wage and buy commodities can give rise to a surplus of value — an overall “growth” of capital wealth — in the economy
This is why wage labor is the source of value in a capitalist economy and why socially necessary labor time is the measure of value for commodities. The surplus of time that we produce makes it possible for us to be exploited in the social form of wage labor, which converts our surplus of lifetime into surplus value for the sake of profit and the growth of capital.