Krishna reaches back to the very origin of things. At the dawn of creation, Prajapati brought beings into existence along with the principle of yajna. He told them: through this spirit of offering and mutual service, you will flourish. Yajna will be your Kamadhenu — the cow that grants every wish.
Kamadhenu is not a magic trick. The idea is that when human beings live in a spirit of giving — offering their effort, sharing their resources, serving the larger whole — their own needs get met naturally. The universe operates on reciprocity. This is the foundation that Krishna is laying before he explains the full cycle.