For different times of day in one render pass (it's intended for use with LightMix). Sure you could change the color of the sun from noon to evening with LightMix before, but guess what, your sun was still coming down from the center of the sky, not at all like evening ;) Now you can set up different suns at correct angles for different times of day, and in one render get two, three or more different times of day thanks to LightMix.
EDIT or as I always like to say, also for rendering scenes on Tatooine.