Ladies who are dropping off kids at school find them useful I think. It means they know they are in a no parking area or yellow line zone. Even the area marked Do Not Park Bus Only. I know because I have seen it!
'invisibility-shields' for white van drivers? Forgetting about a not insignificant cross-section of the rest of the other road users for a moment,yes,lots of vans do "stop,switch-on,bolt" but its the rigid HGV's that complete their newspaper purchase then drive around the rest of the day with them merrily flashing away I find most disconcerting as I never know just where they're going,left/right or stopped.