Lightning Exposure

I'm playing a shock nova/vaal arc elementalist and I've been playing around with some timeless jewels but I have a (probably stupid) question.

Smite of the Heretical gives 10% chance to apply lightning exposure on hit (tool tip says "Lightning exposure applies -10% to lightning resistance for 4 seconds"). Now my question is, what does that really mean to me? 95% of mobs just go pop in one hit anyway. Is this more for single target and if so what does -10% resistance really mean for my build?