The steam cycle has to be up and running before sunrise so they can meet their generating obligations, so they have to ramp up with natural gas while it is still dark. The facility is also capable of operating as a dispatchable "peaker" power plant.

There's a huge amount of right-wing disinformation about solar power so if you just Google for "Ivanpah natural gas" you'll get an outpouring of stupid blogs, but reading them will get you close to the truth.