The Old Testament is full of signs that pointed toward Jesus. It's hard to fault the people who lived in the midst of these signs for missing the big picture - it's hard to see the forest when you're standing next to one of the trees. We, on the other hand, aren't limited by our context in history. We can look back and see the ways that the Old Testament prefigured the coming Messiah and showed exactly who he would be and how he would do his work.
How does this help us? The more clearly we see that God made direct, specific promises about the Savior and then proceeded to keep them perfectly, the more confident we are the promises he has made to us. This gives us security, hope, and an contagious spirit of contentedness as we live.