Supposedly they have to suspect you of having committed a crime, or preparing to commit a crime in order to make you "Stop and ID." However, probable cause doesn't stop the police from stopping you for no reason. Probable cause is only of use if they stop you for no reason and then you end up being searched without consent or something.

In states that are not stop and ID, you cannot be cited for refusing to stop and talk to police. (This does not hold if you are in a motor vehicle). I was stopped on foot once by a police officer in Idaho (not a stop and ID state) at about 2 in the morning once (coming back from my uncle's house after a party). I told the Bonneville County Sheriff that I was not in the act of committing a crime and kept walking. The guy actually tried to grab me and hold me. I had to ask "what am I being detained for?" He wanted me to produce ID. I told him that Idaho is not a stop and ID state and crossed the street. He let me go after that. The cops are not your friend.

I've had a police officer stop me when I was younger. He wanted to search my car. I told him I did not consent to a search. He tried all sorts of bull about how it would only take a minute and how I would consent if I didn't have anything to hide. "Do you have something to hide? We've had some kids running weed in this area and your car matches a description of a dealer." (Keep in mind that the police can legally lie to you). I stood my ground on no searches. I had to ask him about 10 times if I was being detained or if I was free to go. Finally he told me that he could not hold me on anything. Fun times. Cops are not your friend.