It worked well on leaded cars to remove carbon buildup, especially on carby engines where you simply plugged into the vacuum port in the carby base.
I fitted an oiler kit from Sprint Gas to a friends VH some years ago, filled with water not oil, and after a run of about 100 k's and a couple of water fills, he reported no more pinging.
He already had a quote to remove the head to decarb it so the water injection saved a lot of money.
Don't really think it would benifit today's unleaded cars but I really don't know.
It's a simple principal in that it steam cleans the upper cylinder area.