As the sun makes its way across the sky, the color temperature of its light changes from warm (around 2000K), to cool (around 5500K), to warm again (2000K). Research indicates that the varying light spectrum has a physiological affect on organisms, known as the circadian rhythm. Warmer, dimmer light tends to tell our bodies to release more melatonin (the sleepy hormone), whereas cooler, brighter light does the opposite.
If you consider the sleep/wake cycle, this makes perfect sense for diurnal animals. The diurnal cycle is the ideal for humans: ideal for physical and mental health.
Tunable white light allows us to mimic the natural lighting of the sun throughout the day, thus enhancing and reinforcing the circadian rhythm and associated health benefits.