There have been many cases of car crashes where seriously injured people have been removed from the vehicle but the rescue service has failed to see an unconscious baby or small child. "Baby on Board" signs are to alert the emergency services that a child is in the vehicle.
Not meaning to be funny, but the rest of the world don't care if you have your kid in the car, and when we see a baby on board sign we just think that person is proud of being able to reproduce (still doesn't mean it wasn't an accident) and wants everyone else to know.
Being a dippy woman I was stuck trying to park. Luckily this knight of the road arrived and parked for me.