I've never understood why. If He wanted to forgive our sins, why not just forgive them? Why make His son (or Himself, depending on which you think Jesus was) suffer and die so that He could forgive us?
It's all about a relationship with God as it was in the beginning:
- In the beginning, God walked in the garden with His creation
- Sin entered the picture, separating us from God.
- All throughout the Old Testament, God tried wooing His creation back to Him, but everybody continued to push Him away and sinned against Him. (If you'll notice, the prophets and patriarchs had a close relationship with the Lord in the way that Adam and Eve did, but even with Moses, the Lord wanted to communicate with the people, but the people begged Moses to be their mediator instead).
- The punishment of sin is death. The purpose of the Law was to show us where we sinned, thus bringing us to our knees in repentance and hopes of reconciling the relationship with God. But instead, people lost their hope and passion in trying to fulfill the Law.
- God sent Jesus to accomplish what we couldn't.
- Jesus, being the perfect and last blood sacrifice, forgave us of our sins, all the way back to the first one committed.
- Jesus' resurrection allows us to come to Him through faith in Him.
--To your question of why not just forgive us? God did forgive the Israelites, MANY times, even when they didn't deserve it. But when they got it, they took advantage of it. You don't take advantage of those you truly love.
God wants us to love Him back. He wants us to be with Him for all eternity, but because we WANT to be with Him. That's what makes salvation so confusing...all you have to do is repent and accept the invitation. But as you can see, many still prefer to reject God.