Author 1: Maybelle the Rodeo Gal
Answer 1: After branding, cattle grow horns so that handlers can steer steers. Steering is accomplished by pushing or pulling on the horns in the direction the steered steer needs to go.
Steering can be done one or two handed. Turn signals are optional. Unbranded cattle seem to go wherever they want.
Answer (2): Horns were first attached to steers sometime in the late 1930's when commercial steer ranchers started to move their livstock by truck. At first a group of animals was just herded into the back of the truck, but it was soon discovered that perhaps there was a more efficient method of transport.
It is uncertain exactly who should be credited with the discovery, but somewhere in the American Southwest someone started stapling horns to the heads of the steers so that they could be hung from tracks running the length of the roof of the truck. This saved a great deal of space in the back of the ranchers trucks, and they were able to carry more than three times as many animals at one time.
Over a number of years the steer's bodies began to adapt to the stapled on horns, and their skin and fur began to grow around them. What's more, young steer's soon began to realize the importance of having their own horns, so they began to grow their own so the ranchers would not be inconvenienced.