The main purpose for placing home security signs in the yard is to deter burglars. It lets potential burglars know that there is a home security alarm in the house so they hopefully don’t bother attempting to break in.
The benefit of home security signs is that many burglars will move on as they don’t want to get caught. It is also easy to set them out in the yard. On the downside, burglars also know some signs are fake. This is why it is important to actually have a home security system and keep it active. Some other ways to deter burglars include keeping all windows closed and locked at all times and never announcing a vacation or trip on social media.