What is usually done with treasure taken from a defeated enemy? Why is it significant that the treasure from the dragon's den is buried with beowulf?

I'm pretty sure they usually kept the treasure. They used it to display their power, to use as leverage against enemies and to buy stuff at the Viking mall (I'm just kidding about the last part!). They burn the treasure with Beowulf as a fitting tribute to their last warrior king. Although bleeding profusely from everywhere, Beowulf thought that trading his life for all that gold and treasure made him "well endowed" (he means well-endowed with gold and treasure by the way!!) So, although the Geats really liked their treasure, they let it burn with their hero.