Optical audio vs HDMI

Hi all

I have the Apple TV and recently got a 5.1 Surround system set up. I also needed an optical audio cable to link my TV to the Subwoofer. Then I discovered the Apple TV has optical audio.

1. Why would the Apple TV need optical audio when you have HDMI?
2. Does using the optical audio cable activate all surround sound speakers?
   (Since when using Apple TV, only my front speakers work and not the rear speakers.)

Thanks ! 🙂