We can think of reconciliation in terms of family life and friendship.
That is, that God was in Christ reconciling the world to Himself, not counting people’s sins against them [but canceling them]. And He has committed to us the message of reconciliation [that is, restoration to favor with God]. 2 Corinthians 5:19
Reconciliation is necessary because sin is rebellion against God and results in enmity between God and humankind.
For if while we were enemies we were reconciled to God through the death of His Son, it is much more certain, having been reconciled, that we will be saved [from the consequences of sin] by His life [that is, we will be saved because Christ lives today]. – Romans 5:10
The result is peace with God and adoption into His family.