The peculiar bitterness of all this was that Jurgis saw so plainly the meaning of it. In the beginning he had been fresh and strong, and he had gotten a job the first day; but now he was second-hand, a damaged article, so to speak, and they did not want him. They had got the best of him – they had worn him out, with their speeding-up and their carelessness, and now they had thrown him away! And Jurgis would make the acquaintance of others of these unemployed men and find that they had all had the same experience. (12.14)
Sinclair is applying a classic form of Marxist philosophy in this analysis of Jurgis's troubles. According to Karl Marx, the communist philosopher, in early modes of economic development, people sell things that they have made. The price of these things is based on the materials that have gone into them. So, if you are selling a quilt, you base the price on cost of fabric, needles, band-aids for when you have pricked your fingers, etc. However, in a capitalist system, when you want to profit off objects for sale, you start to factor in the cost of your labor. How long did it take you to make the quilt? How much will the buyer be willing to pay you for that time? As capitalism evolves, what is made totally stops mattering. The price of an object is all about how much the buyer is willing to pay; it no longer depends on raw materials at all.
Once price becomes based on market value, the incentive is for the boss to pay his laborers as little as he can while he charges the consumer as much as they will pay. Thus, laborers who are willing to work a lot of time for a little money will get jobs over those who demand higher wages. The content of the work is no longer important; all that matters is the willingness of the worker to put in long hours without much reward.
To take the example of Jurgis, Jurgis works for several companies and it never matters what he makes – at different points in the novel, he works in a meatpacking plant, a factory for making harvesters, and a steel mill. In Marxist terms, this means that he has become alienated from his work. He is no longer directly profiting from the things he makes; he is simply one small part of a much large profit-making system. He is (more or less literally) a cog in a machine. When cogs get overworked and worn out, they get replaced. (Again, this is all according to Marxist thought – to read more, check out Marx's extremely famous foundational text Capital.)
The unfortunate result of capitalist development is that these cogs in the machine are often people, poor souls like Stanislovas running the lard canning machine or Marija trimming cattle. Once Stanislovas develops his terrible fear of cold and Marija injures her hand with a dirty knife, they can both be easily swapped out for other, less worn-down workers. There is no inherent respect or value attached to the worker in this system. This is why Jurgis can complain that his employers "had worn him out […] and now they had thrown him away!" These systems dehumanize workers, which means they get no consideration when they hit patches of bad luck.