At first, the settlement of the West helped cattlemen, but eventually, it forced them to drastically change their way of life.
Originally, it was the settlement of the West that allowed cattlemen to have a place to settle and to keep their herds. It was the extension of railroads that allowed them to get their beef to markets and it was the Army that drove Indians off the land that they used for their herds.
Eventually, however, the settlement of the West forced changes. As farmers came to dominate much of the West, cattlemen could no longer simply allow their herds to roam. As land became more settled, the cattlemen had to keep their cattle much more penned in. This need (along with the invention of barbed wire) led to the end of the era of the open range and the cowboys who were needed to herd open-range cattle.