Networks fail, and systems retry. Without idempotency, a retried payment request might charge a customer twice, or a repeated webhook might create two orders. Idempotent operations use a unique key or check existing state, so a repeat is recognized and ignored or returns the original result.
Idempotency is essential wherever money, inventory, messages, or records are created by automated processes. It allows integrations to retry safely, which in turn makes them more reliable.