Just a guess:

For distortion, white areas in the image push the pixels in one direction. Black pushes them in the other. Gray doesn't change anything

For texture lighting, white areas are "high" and will be lit, while black areas are "low". Think of it as a topographical map with the lighter areas being the peaks and the darker areas being the valleys, and if the sun (the light) is coming from the left, the shadows will be on the right side of the mountains. If you've ever used Bryce or some other 3D landscape generator and fiddled with terrains, it'll make better sense to you.