Locks raise the cost of bad behavior, which makes it less likely. They can still be quite meaningful to someone who breaks those promises, if that person doesn't have the tools or time to defeat the lock, or is just plain lazy.

I live in a pretty low-crime area. From time to time, residents complain about things being stolen from their cars. Every single time that I've seen, the cars have been unlocked. A thief certainly could smash a window to steal from a locked car, but the thieves around here seem to be opportunistic and won't go that far.

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.