Perhaps to instill in us the understanding of the utter dreadfulness of sin. If death were not required to atone for sin, would we regard it (sin) so seriously ? If one of your children were to merely receive forgiveness (aside from any punishment) when they misbehaved, ... would they regard their misbehaviour as such a serious issue ? Good point. One could also point out that the example of Jesus shows us that suffering, itself, is redemptive. Our suffering is not without meaning, but is rather a way that we are redeemed.
SD