What purpose does the Old Testament have today? I've seen different pastors use the Old Testament to fill gaps and sometimes to override information in the New Testament.  However, the New Testament is supposed to be part of the new deal which is to describe the new covenant post-Jesus.

What purpose does the Old Testament have in Christanity today?  Should it still be part of the Bible?
Well, the Apostle Paul specifically states that the things written in the past were for our instruction, for encouragement, and to give us hope.


  For whatever was written in former days was written for our
  instruction, that through endurance and through the encouragement of
  the Scriptures we might have hope.  Romans 15:14 ESV


Also, the Old Testament is not merely a bunch of rules.  On the contrary, it is filled with stories of God's faithfulness, God's ways, God's purposes, God's power, God's mercy, God's justice, God's plan, and God's heart.

Indeed, even though the ceremonial laws of the temple do not apply directly to us today, the story of the Creation, the Exodus, the Fall, God's grace to Hannah and Rebekah, and many other things all reveal God to us.

We can have confidence that as God has delivered His people in the past, He can do so today.  Since God has answered prayers in the past, He will answer prayers today.  Since God saw the affliction of His people in Egypt, He will see the affliction of His people today.  Since God executed justice in the past, He will certainly do so today.

So, no, Levitical laws do not apply to us today.  We are, indeed, under the New Covenant.  Yet the Old Testament still reveals God to us.  

Additionally, it also gives us the promise of the Messiah and the prophecies that were later fulfilled in Jesus, giving us confidence to its truthfulness.

The New Covenant allows us to know God more fully.  The Old Testament help to teach us who He is.