Most of Central Asia isn't great for agriculture, but it's difficult to conquer unless you are, wait for it, the Mongols. It also lends itself fairly well to herding, and since nomads are definitionally good at moving around, they're also good at moving stuff from point A to point B, which makes them good traders. Plus, all their travel made them more resistant to diseases. One group of such nomads, the Yuezhi, were humiliated in battle in the 2nd century BCE by their bitter rivals the Xiongnu, who turned the Yuezhi's king's skull into a drinking cup, in fact. And in the wake of that, the Yuezhi migrated to Bactria and started the Kushan Empire in what is now Afghanistan and Pakistan. Although Silk Road trading began more than a century before the birth of Jesus, it really took off in the 2nd and 3rd centuries CE, and the Kushan Empire became a huge hub for that Silk Road trade. By then, nomads were being eclipsed by professional merchants who traveled the Silk Roads, often making huge profits.