When you're the only ap on your team, but your team sucks so the enemy team stacks full MR

the bolt part when she dashes maybe

{quoted}

I still don't understand

You build that because LeBlanc W got nerfed badly. Because LB Auto's a lot, especially in the early lvls, the combo is op as fuck. If you use those Items + your kit perfectly you will burst the enemies away with out your new W that deals 100 magic dmg at the max lvl ._.