Whenever Appa or another Ally you control enters, creatures you control get +1/+1 and gain flying and vigilance until end of turn.
Flood the board with creatures—especially Allies and tokens—then use Appa's enter trigger to repeatedly pump your team and give it flying and vigilance, swinging in the air for lethal while staying back to block. You build a wide board in the early-mid game and turn each creature entering into an alpha strike enabler.
Each Ally/creature ETB grants evasion (flying) and an anthem, making blockers irrelevant
Vigilance lets you attack and defend simultaneously, great in multiplayer
Mono-white gives access to cheap go-wide tokens, anthems, and strong recursion
Appa himself is a 7-mana evasive vigilant beater that ends games fast
Seven mana is slow and Appa is a removal magnet you must keep recasting
Anthem buffs only last until end of turn, so a wrath resets everything
Mono-white struggles with card advantage and ramp compared to other colors
Ally tribal support is thin, so you lean on generic white tokens to trigger the ability
Mono-white's premier ramp and card-advantage engine to power out Appa and rebuild.
Add efficient ramp and protection—Smothering Tithe, mana rocks, Swiftfoot Boots, and Teferi's Protection—so Appa lands earlier and survives removal. Lean into recurring token makers and ETB density (Captain's Claws, Hero of Bladehold, Myr Battlesphere effects) to fire the trigger every turn, then top the curve with overrun payoffs like Craterhoof Behemoth and Cathars' Crusade for explosive, repeatable lethal swings.