I am a completely newbie to photography. Can someone explain what is special about the GoPro Hero 7? When will it be better than my iPhone 7 Plus camera?
Different tool, different purpose. A gopro is a so called action camera, designed to be easy to operate, not need attention during recording, easily mounted on your body or sporting equipment or on a vehicle, and resilient under harsh environmental conditions. Also, designed more towards the expectation to mostly record video, not still photos.
Most smartphones are not meant to be water, dirt and shock proof, and will need more attention to operating them when recording video or taking photos.