Darth Vader's suit gives him the presence he needs to be one of science fiction's most iconic villains. He is tall, imposing, and expressionless -- a frightening figure even before you hear him speak or see him act.
Dressing a villain in all black is one of the most basic symbols there is, since the light/dark, black/white dichotomy has long been a symbol of good versus evil in Western literature. But the symbolism of Darth Vader's suit goes beyond the basic "black equals Sith"; it reveals important things about Vader's character and the nature of his relationship with the dark side.
Man vs. Machine
As Obi-Wan Kenobi describes Darth Vader in Return of the Jedi: "He's more machine now than man, twisted and evil." The suit does not just support Vader's life; it takes away all outward signs of his humanity. He is faceless and expressionless; the only signs of life are the blinking lights on his suit's front panel and the constant sound of his suit's respirator breathing for him. The glimpse of the back of his head in The Empire Strikes Back is the first confirmation that Vader is not actually a robot.
The struggle between man and machine is a common theme in science fiction, and here Vader's replacement limbs and life support suit represent how by becoming evil, he has become less human. It means more than that, however. In Legacy of the Force, Lumiya explains that losing parts of your body means losing part of your connection with the Force. Vader is still a powerful Sith Lord, but not as powerful as he could have been.
Isolation from the Universe
The Sith view themselves as the center of the universe. Everything and everyone else is useful only to fulfill the Sith's own selfish desires. Isolation reinforces the notion that the self is all that matters. Palpatine chose Sith apprentices who were isolated from the rest of the galaxy: Maul, whom Palpatine hid him away at a young age, and Tyrannus, whose aristocratic background and skill in the Force gave him a sense of being above everyone else.
When Vader first turned, he felt isolated emotionally, rejected by a Jedi Order that did not understand his skill or his passion. His suit makes him literally isolated from the rest of the universe -- unable to touch or interact with anything except through a filter. The suit becomes an outward expression of his feelings of rejection and focus on the self.
Caged in Evil
Most Sith wear black robes, both in the Star Wars films and the Expanded Universe. But these robes are no more than a temporary costume, even for a life-long Sith. Darth Sidious removes his robes to disguise himself; other Sith remove their robes to turn back to the light side. The black robes are a symbol of darkness -- but one that can be cast off at will.
Vader's suit is much more complicated than a simple Sith robe. It is a life-support system, one that Vader cannot remove without killing himself. When Luke confronts Vader for the second time, he is certain that Vader has good in him, and he is right. But Vader is so surrounded by evil that he cannot break free until he is about to die. Ultimately, he returns to the light side of the Force by accepting his own mortality. Letting go of the suit symbolizes letting go of the fear of death that caused him to turn to the dark side in the first place.