"Need" how? The manga seemed to get by fine with depicting her as a human-form robot. Why couldn't the live-action version do the same?
Not the same at all. Except for when she's breaking parts off, the Major visually passes as fully human. Gunm's cybernetics aesthetic is much more openly robotic, and strapping all of that on as prop costume wouldn't be feasible for someone trying to have more range of motion than Robocop.Sol is Major Kusanagi, and not using CGI wasn;t the reason GitS live-action was bad.
Like I said before, I'm not arguing that her facial depiction isn't weird, especially considering the trailers show other full cyborgs except for the head with fairly normal faces, but the CGI useage isn't surprising.