Networks are unreliable, so a request may be sent twice when the first reply is lost, even though the action succeeded. If that action charges a card or places an order, repeating it is harmful. An idempotent operation is designed so that doing it again has no additional effect beyond the first time.
This is often achieved by attaching a unique key to each request, so the system recognises a repeat and ignores it. Designing operations this way turns a frightening uncertainty, did it happen once or twice, into something a client can safely retry.