The idea is that it'll make people more cautious drivers around you. It doesn't work because people generally transform into sociopaths the moment they enter a vehicle.

they're warning signs for other people that the driver of the signed vehicle is likely not paying attention to driving

or at least that's how it appears from my experience, given that there's a fucking good chance that a car with a sticker like that does something really stupid like change lanes without signaling or giving a shit that I'm right beside them