Why the Cross?
Where did Jesus go for 3 days after dying?
Actually, Jesus didn't remain in the tomb. he descendS into hell.
The death of Christ and His resurrection were the reconciliation between God and man.
When 1st Adam introduced death into the world by sinning, the 2nd Adam (Jesus Christ) defeated death at the cross, because He died as a sinless man, giving mankind access to eternal life.
That's why He is called - The risen King
For 3 days, He went into Hades (Sheol/Hell), in warfare, to take the keys from Satan (who held the keys of Hell), set captives free (all the righteous souls that were there, starting with Adam, continuing with Moses, David, Elijah, Daniel, Isaiah, all the Prophets and many more) and to break the chains of death, that death itself had on humanity.
That's why the work on the cross (which is the greatest battle ever fought and won) was and is crucial for mankind - for we now have access to Heaven through Jesus - He has the keys of death now, not Satan, and that means only one thing: if we follow Jesus, as our Lord and Saviour, we can have eternal life, for He is the Truth, the Way and the Life.