Interesting that several have mentioned stopping a person because of "a reasonable belief that a crime is about to happen."
I find that kind of troubling---in that it suggests the law enforcement official can predict behavior.
I am not sure how one would have a reasonable belief of what someone has in mind.
The fact that I go into a bank with a CCW gun does not mean that I plan to rob it. But I might.
I suppose what you guys are saying is that it gives the law enforcement official the right to check me out to see what my intention is.
But if I am planning to rob the bank and I get asked about my intentions by an officer, surely I'd deny it.