Determinism.
A spend either succeeds for every observer or fails for every observer, with no global-state surprises.
Like calculating a chess move on paper before playing it. The board does not change while you think; the move plays out exactly as you computed.
In depth
Because eUTxO validators see only the transaction at hand, the outcome of submitting a transaction can be computed in advance. With honest pre-validation, transactions never fail on-chain unexpectedly. If a Plutus script does fail in phase-2 validation, only the dedicated collateral input is consumed, not the full transaction value. Compared to account-model chains where contracts can fail mid-execution and burn the full gas budget, this is a clear safety win.
Example
Because the validator only sees the transaction itself, a Cardano dApp can simulate your DEX swap locally and predict whether it will succeed before you submit it.