"The gospel was preached even to the dead." The descent into hell brings the Gospel message of salvation to complete fulfilment. This is the last phase of Jesus' messianic mission, a phase which is condensed in time but vast in its real significance: the spread of Christ's redemptive work to all men of all times and all places, for all who are saved have been made sharers in the redemption.
Source: Catechism of the Catholic Church (Vatican.va)