Actually, that is an excellent question, Maize.
One site 'thestillsmallvoice.com churns out the usual:-
The Bible tells us that "Christ Jesus came into the world to save sinners" (1 Timothy 1:15). God is a just God who must punish sin. The Bible says that "the wages of sin is death"; and that "all have sinned, and come short of the glory of God" (Romans 6:23; 3:23). In order to save us sinners and make us acceptable in God's sight, it was necessary for God the Son to become a man and live a sinless and holy life in our place, and then to suffer upon the cross the just punishment for our sins and the sins of the whole world (cf. Romans 3:24-26; 5:6-10; Ephesians 1:6-7). This Jesus Christ has done for us! Because of His innocent sufferings and death in our stead, God is gracious and merciful to us sinners. Through faith in Jesus and His cross, we have forgiveness for our sins and everlasting life in heaven instead of spiritual death and the everlasting fires of hell which we deserve. When Jesus was about to die on the cross, He said, "It is finished" (John 19:30). Jesus had accomplished our redemption; He had paid in full the punishment for your sins and my sins! It was necessary for Jesus to die upon the cross to save you. He died for our sins and rose again. Hallelujah!,