Since God provided for the forgiveness of sins in the Old Testament, why do we need Jesus?

I thought of this when I was researching [this question](https://christianity.stackexchange.com/questions/3121/when-did-knowledge-of-christs-sacrifice-become-necessary-for-salvation) . [Leviticus 4](http://www.biblegateway.com/passage/?search=Leviticus%204&version=NIV) goes into great detail about the sacrifice that must be made in order for a sin to be forgiven. However, we see here that God actually provided a way for these sins to be forgiven. Specifically, Leviticus 16 shows that this sacrifice and the [Day of Atonement](http://www.biblegateway.com/passage/?search=Leviticus%2016&version=NIV) did provide forgiveness of sins: >[Leviticus 16:30 (NIV)](http://www.biblegateway.com/passage/?search=Leviticus%2016:30&version=NIV)Emphasis added
>because on this day atonement will be made for you, to cleanse you. **Then, before the LORD, you will be clean from all your sins.** Now, Jews no longer provide sacrifices for the forgiveness of sins, however God has still provided a way for these sins to be forgiven. (See [Why don't Jews sacrifice animals anymore?](https://judaism.stackexchange.com/questions/8862/why-dont-jews-sacrifice-animals-anymore) for more information.) So, if God has previously given us a way to have forgiveness of our sins, why did he send Jesus to be our ultimate sacrifice? *Edit:*
I believe that there is some doctrinal basis for this. I'm not sure which specific doctrine would best be applied to this, but I am seeking a mainstream Protestant doctrine on the matter.