You want someone who is proficient with using a camera, who has expertise shooting pictures in various lighting and environmental situations, which can compose various images and execute them flawlessly. The photographer performs this instead of the camera and they have honed their expertise and knowledge through many hours of shooting and years of experience. With this, you get stability and an entire album's worth of gorgeous photographs, not just one or two. Professional photographers have years of experience photographing weddings and are knowledgeable about the best angles to use.