Avacyn, Angel of Hope makes your entire permanent board much harder to destroy. She has indestructible herself and grants it to your other permanents, protecting creatures, artifacts, enchantments, and lands from destruction effects. Flying and vigilance also make her an imposing attacker that remains available to block, giving white decks a defensive engine that can close a game through combat.
Wrath of God becomes much more favorable when Avacyn is protecting your creatures, since opposing creatures without protection are destroyed while yours remain. Angel decks can support her with creature-type synergies, and reanimator decks that include black can bring her back with Reanimate or Animate Dead rather than paying the full eight mana.
The triple-white requirement makes a reliable white mana base important if you intend to cast her normally. Mana rocks and land-based acceleration help reach that point, but it is often worth keeping some protection available rather than spending every resource to deploy one large creature. Exile, bounce, sacrifice effects, and sufficient toughness reduction can still remove her.
Think about which existing permanents her arrival will preserve. Protecting a developed board of engines and attackers usually provides more value than casting her onto an otherwise empty battlefield. Once she is in play, vigilance lets you pressure an opponent without giving up your largest blocker, while the team-wide indestructible effect allows more confident attacks into ordinary combat damage and destroy-based removal.
