L + drag rotates a light (relative from its current position) in the direction you drag.
Rotating the light source is useful when you want to backlight a model and is useful when you need to rotate an Image Based light source.
L + right-drag rotates the light so it immediately points from the location you click in the 3D View.
This option is useful if you frequently tumble the model when sculpting and want to quickly update the light’s direction to evaluate the model’s surface.