This verse is a great description of the transition from the Old Covenant to the New Covenant and why Christ had to die.
Heb. 9:15 For this reason He is the mediator of a new covenant, so that, since a death has taken place for the redemption of the transgressions that were committed under the first covenant, those who have been called may receive the promise of the eternal inheritance.
In the Old Covenant (“first covenant”) there were two covenant partners, God and Israel. To breach the terms of this covenant was deserving of death. Now, all of Israel sinned against God, yet, many sinners of old trusted God and were counted righteous in spite of their sins. So before God could establish the New Covenant, God had the outstanding debt of death from the transgressions under the Old Covenant. Therefore, Christ had to die to pay for these deaths before the New Covenant could be established. Christ satisfies the terms of the Old Covenant by dying for sinners and in doing so, He can now establish and be the mediator of a new covenant (Jer. 31:31ff).