I once heard a minister tell an audience, "The Old Testament isn't relevant to our times so there is no need to study it anymore."
How wrong he was! One reason I love reading the Old Testament is because it explains the New Testament in clear, simple terms. In the Old Testament, for example, Israel is a type of the church and Egypt represents the world. Israel's journey through the wilderness represents our spiritual work as Christians. Also, the tree that healed the waters at Marah is a type of the cross of Christ and the rock that produced water in the desert is a type of our Savior.
Scripture makes clear that all of Israel's physical battles mirror our spiritual battles today: "All these things happened unto them for ensamples: and they are written for our admonition, upon whom the ends of the world are come" (1 Corinthians 10:11). Even the tabernacle and its furniture are examples of heavenly things: "Who serve unto the example and shadow of heavenly things, as Moses was admonished of God when he was about to make the tabernacle: for, See, saith he, that thou make all things according to the pattern shewed to thee in the mount" (Hebrews 8:5).
All these Old Testament examples are meant to keep us from falling into unbelief, as Israel did. The author of Hebrews writes, "Let us labour therefore to enter into that rest, lest any man fall after the same example of unbelief" (4:11). In other words, "Study the Old Testament and learn from Israel's example. Do not make the same mistakes they did!"
Whenever I don't understand a truth in the New Testament, I turn to the Old Testament to find it illustrated in some way. For example, let's say I want to learn how to bring down spiritual walls the devil may have built up in my life. I turn to the story of Joshua to see how the walls of Jericho were brought down. Israel's physical battle with those walls provides me with a picture and a pattern, to help me understand how I can bring down all the walls that keep me from attaining fullness in Christ.