The practical effect that my coworkers keep telling me about is that the client or an intermediary like a load balancer may retry the same request until it is successful, with the expectation that the resource (records or settings in a database system in our case) is only modified once. A database query that does something like value = OLD.value + 1 cannot be safely retried. There are circumstances under which the outcome of a transaction can be unknown but it might still succeed, and the client should be prevented from doing accidental changes. With read queries this should be rare, but even then there can be side-effects affecting the overall system state.