I believe the intent is to make Tanks and Enchanters more relevant overall by improving their ability to contribute in teamfights.
Because which game has more damage in it:
Duelist toplane
Duelist Jungle
Burst Mage Midlane
ADC Botlane
Mage Botlane
vs
Duelist toplane
Duelist Jungle
Burst Mage Midlane
ADC Botlane
Mage Botlane
or
Bruiser toplane
Tank jungle
Control mage midlane
ADC Botlane
Enchanter Support
vs
Tank toplane
Duelist Jungle
Burst Mage Midlane
ADC Botlane
Tank Botlane
Half the reason there's so much damage is that we're typically seeing all damage vs all damage comps. If they improve tank contributions somewhat, they can bring more tanks back into the game, thus reducing the amount of damage TOTAL even if they increase the amount of damage the tanks themselves bring.
that being said, I am NOT saying this rune is necessarily the best way to do that... but the theory is sound, to a degree.