These monsters benefit from having more monsters in the Graveyard to be banished. Dark Necrofear can easily be summoned if you have milled out at least 3 DARK Fiend monsters. Tyranno Infinity gains massive ATK boost if you have milled out a lot of Dinosaur-type monsters and banish them from your Graveyard. Gaia Plate the Earth Giant can easily be Special Summoned and maintained with a substantial amount of Rock monsters being milled out. Prometheus, King of the Shadows can proceed to banish any number of DARK monsters from your Graveyard to greatly enhance its ATK until your end phase. Chaos Necromancer ATK greatly increases with the number of monsters in your Graveyard.
Powerful Rebirth brings back important monsters from your Graveyard to the Field. Anti-Magic Arrows prevents your opponents from disrupting your attacks by flipping their face-down Spell/Traps.
Could be used to mill Spellbook cards to your graveyard and fuel the effect of Spellbook of Fate. Spellbook of the Master also greatly benefits giving you an assortment of Spellbooks in the graveyard whose effects you can copy. Could Fulfill the activation condition of Spellbook Library of the Heliosphere by loading your graveyard with Spellbooks.
~this is from Spellbook library card text~
"You can only activate 1 "Spellbook Library of the Crescent" per turn. You cannot activate any Spell Cards the turn you activate this card, except "Spellbook" Spell Cards."
This card is obviously extremely powerfull if anything speed duels buff the grass as 30 card decks can open it much more consistently than their 60 card counterpart my guess is that if the meta shifts more towards graveyard effects konami will have to ban it or release a skill which plays necrovalley at the start of the duel