From launch until Cataclysm, holy paladins survived on just a handful of heals. We traded healing complexity for unique utility, gaining Aura Mastery instead of Wild Growth or using Hand of Freedom instead of Prayer of Mending. Holy paladin healing was so simple for so many years, as we tended to gravitate toward a single spell like Flash of Light or Holy Light in each expansion. While any other healing class could have complained that their class had become stagnant, holy paladins had literally been playing the same game of whack-a-mole for years.
Cataclysm's massive retooling of paladins and introduction of the three-heal model helped breathe new life into playing holy. Holy power gave us a unique mechanic to manage and gave us additional choices to make when healing. The three-heal model ensures that we're forced to choose between throughput and efficiency, and now we're challenged with managing our mana as well as our healing. Even though these changes drastically shifted the way we play our paladins, we were still relying on the same single-target heals we had always been. Healing Hands, now known as Holy Radiance, was the spell designed to upset the status quo.