Immanuel means "God with us."  Why did God have to become a human being?  Well, both Jews (who had the Law) and Gentiles (who did not) had sinned and thus were guilty before God.  Sin brings death so this situation left the entire human race in a hopeless mess.  But God's plan for mankind's salvation (which was His eternal purpose and plan from the very beginning) was for Him in the person of Immanuel to overcome death by living a sinless life and offering Himself as the perfect sacrifice for sin.  By man came sin and death, so by Man had to come perfect obedience and life.