There are three principal reasons that Old Testament Survey is important for missionaries and all Christians. First, the Old Testament makes up the first 3/4ths of the Bible. Second, the Old Testament was the Bible of Jesus and the apostles. Third, the Old Testament is about Jesus. The first two of these assertions are beyond dispute, the last, however, deserves some explanation.
Clearly the New Testament is about the person and work of Jesus Christ. Equally clear, however, is that Jesus declared to the Pharisees in John 5:39,
You search the Scriptures because you think that in them you have eternal life; and it is they that bear witness about me.