Film grain is similar to noise which by definition compresses extremely poorly. By actually modeling the way the film grain is produced, they can avoid sending

Film grain is similar to noise which by definition compresses extremely poorly. By actually modeling the way the film grain is produced, they can avoid sending what is essentially random data over the wire while still preserving the visual effect of the film grain itself. That way, the bitrate can be spent to encode the actual video data at a higher quality.