Verbal boundary enforcement communication. It is letting a would-be assailant know that you are aware of what is occurring and that you are committed to doing whatever it takes to protect yourself.
This is not threatening nor blustering; it is a clear cut message for him to "get off mah land." And that failure to do so will have unpleasant results. This is being assertive, not aggressive. You are doing what it takes to get him out of your boundaries, not chasing him down the street. Before you vocalize: Know how far you are willing to go