Why are differing Kelvin ratings necessary at all? In a word, DEPTH! The natural reef environment begins at the surface and can reach to depths down to 150 feet, or more!.
If you have ever snorkeled or scuba dived, then you're aware of how quickly water depth can effect light intensity. The lower one goes, the dimmer and darker the light, turning from turquoise to azure to cobalt in very quick order. This obstacle haunted the hobby in it's infancy until Philips and other tube manufacturers realized that there was a dollar or two to be made in this hobby! Once it became economically feasible, tubes strictly for our tanks suddenly began appearing in magazines and our stocklist shelves. Holy reef tank Aquaman, I can now keep that once delicate specimen thriving in it's little glass box for more than a month or two!
Seriously, the K rating, or CRI, gives you the ability to match the tank lighting to that of your animal's natural habitat. Without this important advances in the industry we could never hope to duplicate lighting conditions at DEPTH in our living rooms, and provide as near identical conditions for our corals and inverts as that in nature.