SQL databases provide ACID guarantees:
Atomicity: All operations in a transaction succeed or all fail. No partial updates.
Consistency: The database moves from one valid state to another. Constraints are enforced.
Isolation: Concurrent transactions don't interfere with each other.
Durability: Once committed, data survives crashes.
ACID matters for financial systems, inventory, and anything where partial updates cause problems. Transfer : debit from A AND credit to B. Both must happen, or neither.