The best rule of thumb when it comes to determining when to dim high beams is that a driver should turn headlights to low when he sees another car's headlights, either in the rear view mirror or coming toward him. Laws regarding the matter vary by state.
In the state of South Carolina, a driver must dim high beams if another car is oncoming and 500 feet or closer or if he is closer than 200 feet behind another car, as of 2013. In Washington state, the law is 500 feet or closer for oncoming traffic or 300 feet behind another car.
Putting on high beams is helpful for drivers who are on a road or highway without lighting. The elevated beams help drivers see farther down the road, increasing their ability to react to such hazards as sudden turns in the road, debris that may have fallen off another vehicle or animals that dart out into the road.
However, drivers coming the other way or in front of a car get the light from high beams right in their eyes, either directly or from their own rear view mirrors. This temporary blinds them, leading to a danger of collision.