I know that the original tale of Beowulf was told before it was copied down with Christian references, so that as pohnpei397 notes, the idea (as with the Egyptian Pharaohs) that one might needs his treasures after death might promote the idea of an afterlife.
However, the version of Beowulf that has survived has clear references to Cain the murderer, and Grendel's descent from the line of Cain (showing a Christian influence). We could also infer that Beowulf is buried with the treasure to remind him in his last moments of the things he achieved in his life time that he would be leaving behind.
At the end of the story, it says:
Yet it was not greed for gold, but heaven's grace that the king had ever kept in view.
This shows that Beowulf the warrior cared more about an afterlife in heaven than for treasure.
He also thanks God for enabling him to win the treasure for his people:
I give thanks to my God, to the Wielder-of-Wonders, for the gold and treasure upon which I now gaze; I thank Heaven's Lord that I have been given grace to acquire such for my people ere the day of my death has come! Now have I bartered the last of my life for the hoard of treasure, so look well to the needs of my land!
His wishes are to care for his people before he dies. The fact that he speaks at such length about God's blessings for his people—through him—makes me believe that he will not embrace simply the sense of God's blessings, but that as he dies he will also have the hope of a life after death. It doesn't make sense that the text influenced by Christianity would leave that detail out.