The only way I've seen flashing high-beams used is on the highway to signal from behind "you're going too slow for my liking" or from a distance away "watch out I'm serpentining through traffic 15-20mph faster than the next fastest car"
I think GP is talking about flashing the headlights by turning them off and back on quickly, rather than flashing the high-beams. This is what I've usually seen in the US when drivers want to say "thank you" or "you can merge in front of me" on the highway.
Ah, ok. Yeah, that's an important distinction. I've actually not come across that as of yet.