The biggest difference is line of sight communications can not be picked up by a microphone. No need to worry about smartphones, or talking too loud and neighbors hear it. Even in public, you can find a blind spot where CCTV doesn't see you, then use sign language, even a cop around the corner cannot know what you signed, but if you talked, they could hear your prescense. In many situations, it's much easier to control who can see you, vs controlling where your soundwaves go. Short of any reality-bending inventions, physics is physics, light always stops with a wall. This isn't just speaking a different language, this is a whole different approach to how your communicate.

I mean, you know how when a covert ops military operation is depicted in media, the soldiers always use sign gestures when around enemy troops? It's like that. Harder to detect.