The protection is useful because for a highway rear-end accident to occur, a chain of at least two events has to occur. The car ahead of you has to significantly decelerate, and in the same window of time, the rear-ending driver has to be distracted for longer than the time it takes to close the gap and stop. Most of the time both of those events don't occur at the same time, but just look at traffic accident reports to see that in areas with heavy traffic - the probability that both events occur over the population of cars in traffic is 100%+ (because it happens multiple times every day).

Adding some sort of auto brake feature adds another layer of safety (presuming that the ratio of 'saves' to added risk is very high with this tech.)