Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This build is for magic penetration. You also start the game with +180 health item, and the +120 from runes, meaning you can solo your lane very well. Pop clarity for staying power in the lane you're soloing, and save your heal for emergencies.
If in a 5v5 group and theres 3+ casters in the group, I use this build because I know the enemy will stack some magic resist. If in a 3v3 with 2+ casters, I use this build too.
At the end of the game you have insane AP, so use Requiem wisely, finishing runners.