The Old Testament is designed to prepare us for Christ, and if nothing else, the messiness, the sin, and the squalor of the Old Testament should convince us of man's need for Christ.
This preparation is not merely an intellectual readiness to accept the New Testament. This would be difficult for us all because our minds are dense. It is also not a one time preparation that dissipates in meaning once the New is embraced.
It should grind away at our encrusted hearts. We read with horror at the murders, the adulteries, the idolatry and sit in stunned silence. If the silence is allowed to work, we can see beneath our veneer and find an inner alliance with those who walk the ancient pages.
Despair may grip our heart so that we cry out to God for salvation. Only then can the light of resurrection come. This is the work of the Old Testament. It is the cross of the New Testament's resurrection.