Hmm. That makes sense, but when I've used it, it hasn't moved to another field. That could be because I haven't used it when there were more than one available field though. I never paid that much attention.
The way I've usually used it has been to force words I type to stay that way and prevent the keyboard from auto-correcting them to something else. Once I finish typing the word the keyboard wants to auto-correct if I type a space, I hit that arrow key and it moves ahead leaving the word as I typed it. It's always seemed to work for preventing auto-correct, so I thought that was its purpose. If not though, how are you supposed to prevent words from getting changed by auto-correct?