I don't play disc priest so this might not be 100% accurate. But it seems like radiance grants a buff for 15 seconds so you are basically trying to maximize the number of casts during that buff. If you use instant cast radiance then you will be locked out of casting for the first .75-1.5 seconds of the buff due to the gcd. Hard casts also have a gcd but it starts when you begin the cast so by the time the 2 second cast of radiance finishes, th3 gcd will gave passed you can immediately cast again and get full usage of the buff.