In this featurette from the Lucasfilm video archives, sound designer Ben Burtt discusses the evolution of Darth Vader's iconic, ominous breathing.
In the original script, Darth Vader was described as a "strange dark being, in some kind of a life-support system." As a result, Burtt originally created many sounds for Vader: his heart audibly beating, gears turning as he moved, and more. This turned out to be too much, however, and was scaled back to just his mechanical lungs -- which became a classic movie sound effect, and remains a defining feature of the Dark Lord of the Sith.