FlyBry:

I'm trying to remember if this is done ICAO-wide, or just in the US. Been too long since I've flown International or on a heavy.

Regardless, the intent is to inform other aircraft on-frequency that the aircraft is likely to have very strong wake vortices. ATC should provide adequate separation, but pilots are allowed to ask for additional separation.

The B-757 has probably the worst vortex of any airliner, and models that do not meet the 255,000 lb weight category are still called "Heavy" on the radio.