The right mountain bike handlebars can help you steer and maneuver more precisely, maintain a proper riding position, and even absorb some of the impact and vibration from a rough course or trail.
One hundred years ago, there was one basic bicycle type: frame, two wheels, narrow tyres, rim brakes, saddle, and handlebars. Nowadays, there are disc brakes, flat bars, knobby tyres, folding frames, ...