There was a study done a while ago on perceptions of threat. A lot of our driving/riding techniques are actually avoidance responses to perceived threats - oncoming traffic, objects on the road etc. Apparently when we're on our feet we perceive objects that're taller than they are wide as a threat (eg: a well built tall person is more of a "threat" than a well built short person) but when we're driving threat perceptions change to objects that're wider than they are tall (eg: other cars are more of a "threat" than bikes).
Because the headlight is the most obvious part of a bike, putting driving lights on makes it appear wider and more "threatening".
