Stoneskin

The +0/+10 is not a fight-winning buff; it is a combat-math eraser. Toughness that high lifts the enchanted creature out of range of any single burn spell or damage-based removal you are likely to see, and it turns a chump block into a wall that absorbs a swing and lives. Flash is the part that makes this a trick rather than a pre-combat commitment: you hold it up, let the opponent commit to an attack or point a Lightning Bolt at your blocker, and only then decide whether to spend the mana, flipping a favorable-looking trade into a dead card for them. The cost of that safety is the classic Aura tax. It does nothing on offense, adds no power, and hands the opponent a two-for-one if they can bounce or destroy the creature in response (and toughness buys nothing at all against deathtouch or edict effects, which erase the body regardless of how tall it stands). Pumping toughness alone is one of white's oldest defensive levers, reaching back to the earliest protective Auras; this is the flash-enabled, no-power-added distillation of that idea, pure survivability sold at instant speed. It wants a board where a big-toughness body actually matters (a blocker you cannot afford to lose, a creature carrying counters or another aura worth protecting) and asks nothing of your deck otherwise.