The short answer is: they're for other people.
Here's a practical example: I drive a black car. In the late summer, the trees around my neighborhood really overhang the road and block a lot of the lower-angle sunlight. Also, the roads near my house range from 10-20 residential roads to full highway speeds.
So, at a glance, my car doesn't passively give off a lot of visual cues about its orientation using the decreased ambient light. If you look, you can see that I'm definitely going 20, 40 or 60 mph but it might be just a little less clear out of the corner of your eye.
With the running lights, it's just a little more obvious. The human eye is great at picking up contrast and motion in the rod-dominated periphery of your vision.
All that said, they aren't a magic spell: if, say, you happen to be the kind of person who's talking on your cell phone, looking the other way while you cut me off in a parking lot (just to pose a not-so-hypothetical example), there's nothing that a couple of little lights on my car can do.