Why do the packinghouses hire extra employees? How does this excess of workers impact Jurgis?

From the text:

"During the summer the packing houses were in full activity again, and Jurgis made more money. He did not make so much, however, as he had the previous summer, for the packers took on more hands. There were new men every week, it seemed--it was a regular system; and this number they would keep over to the next slack season, so that every one would have less than ever. Sooner or later, by this plan, they would have all the floating labor of Chicago trained to do their work. And how very cunning a trick was that! The men were to teach new hands, who would some day come and break their strike; and meantime they were kept so poor that they could not prepare for the trial!"