I'm not entirely sure about the motive of baby-on-board other than potentially "if there's an accident look for the baby". I don't think it's an excuse to be a bad driver but hopefully encourage people around possibly to be mindful

I've seen quite a few stickers on cars that basically state there's an autistic child/person in the car and they have a list of things the passenger may exhibit such as being non-verbal, reacting strangely, running away... Like on some houses they list occupants human and otherwise in case of emergency and people and pets need to be rescued