Not very scientific, but It seems to me that having a triangulated light pattern makes it easier to judge speed and distance for the observer. Look at the photos in this thread and notice that with the auxiliary lighting, there are now three or more focal points rather than just the single headlight. Our brains tell us to pay more attention to lights over background, and once noticed, we start processing velocity. Railroad safety engineers seem to agree. A single beam on a locomotive is more difficult to judge closing velocity than the light triangle common today; even better than the sweeping or oscillating beam used in the 50's and ealrly 60's.