Emphasizing character transformations of Sarah Connor and the T-800
The Terminator wears sunglasses to create a parallel when Sarah wears them later:
James Cameron (writer / director / producer): We wanted T-800 to change, and we wanted Sarah to change; she starts out one way, becomes fixated on that, ends up becoming more of a machine that he does. We wanted the two of them to change characters as the film went on: she becomes the Terminator while he becomes a human being. And it's partly through the Terminator's transformation that she understands what humanity really is.
Mali Finn (casting director): Jim Cameron's movie (T2 ) does have all of those special effects but still is a character driven piece and has wonderful characters. We've got to have good actors as they're playing against all the special effects. That’s what distinguishes his action movies from other people’s action movies.
Mario Kassar (executive producer): He's a great storyteller who can always take a plot to much higher levels. As the movie progresses, Sarah becomes the Terminator. The sunglasses and use of red dot for targeting accent the transition visually.
As Cameron himself explains, there are two transitions occurring: over the course of the film, Sarah becomes the Terminator and the T-800 becomes a human being — both of which are symbolized by the sunglasses (or lack thereof).
The sunglasses are an important story device in The Terminator and T2. In the first movie they meant to dehumanize T-800 more and more as the movie progresses and as he takes on more damage, becoming less and less human-like in appearance. In T2...the sunglasses have an even deeper meaning and message. The sunglasses symbolize his gradual transformation — he wears the sunglasses in the beginning when he is nothing more than a killer with a blank mind and loses them when he's starting his journey to become more human...
(Source: James Cameron Online, "T2 Complexity")