I'm fairly certain that reduced noise is just a beneficial side effect and that the original motivation for continuous welded rails was less wear and tear, less

I'm fairly certain that reduced noise is just a beneficial side effect and that the original motivation for continuous welded rails was less wear and tear, less required maintenance and better ride quality. It's also a necessary prerequisite for running at higher speeds, because a rail joint can only be subjected to so much forces before it starts failing at an accelerated rate, and dynamic impact forces while transitioning a rail joint naturally increase with speed.