Each element is tied to a certain enemy faction. and their shields is that element. using the element on them deals bonus damage and helps take down the shields much faster. Fallen - Arc damage, signified by blue shields Hive - Solar damage, signified by orange shields Vex - Void damage, signified by purple shields (exception: some Harpies have arc shields) Cabal - Solar damage, signified by orange shields Most of the time you won't need the element to deal damage, but in a few rare cases (I've noticed one area so far with Vex Harpies with Arc shields that only take damage from Arc attacks).