Cowboys on open range control the herd so they don't stampede, defend against predators and rustlers, and move the cattle along at a rate of speed that keeps them together, but doesn't burn any more calories than necessary getting them to market (more they weigh, the more they're worth).
In open range, they have a round-up to brand the calves with the ranch's symbol to show ownership. When cows are calving, they make sure everything is safe for the mother, and help with the birth if the calf is facing the wrong way.
From time to time they "cut" or separate the cattle, whether for market weight calves, feeder calves, "shadow" calves, or for maintenance (cyclic shots, branding, neutering, etc.)