It's a lot like turn signals - social communication that goes beyond the practical benefit. If you're using your turn signals, you're saying "I'm aware of the environment and a good participant in the game we're playing because I'm following the rules". If you don't use signals, you're telling people that you're not following the rules, and that makes you suspect in all the other social games. Kinda funny to do some people watching with that perspective, and to start to see how many assumptions are based on society being high trust - the exploitable vulnerabilities are endless, and people communicate a lot about themselves in the rules they choose to follow or break.