A treasure hoard usually symbolizes spiritual death/ damnation in Old English lit. How does this add significance to Beowulf's last fight with the dragon?
This fact suggests different interpretations. When the dragon's hoarded treasure is violated, the dragon inflicts punishment on many, not just the thief. The hoarded treasure, then, and the evil that came of it could indeed symbolize spiritual death and damnation. By helping Beowulf slay the dragon, Wiglaf and Beowulf together overcome this evil. Both warriors act in a way that is morally consistent with their cultural values and religious beliefs.
Beowulf, mortally wounded and knowing his own death is imminent, commands Wiglaf to build a tower by the sea to hold Beowulf's remains and to stand as a monument to him and a guide to sailors coming over the ocean. Beowulf further commands that the treasure be buried in the tower, not shared by his warriors as was the custom. Beowulf's men, with the exception of Wiglaf, had forfeited their right to the treasure by betraying their king in the last battle. When the dragon had defeated Beowulf and was about to kill him, only Wiglaf fought to save his king. The others ran away in cowardice. The treasure, then, also could be interpreted as a symbol of the warriors' betrayal of their Christian duty (as the epic came to include elements of Christianity) and their heritage as Geats.
If we agree that the treasure hoard in Beowulf is meant to represent a spiritual death or damnation, then several interpretations of significance present themselves. One of the most obvious is the notion that, if the treasure does represent damnation, then Beowulf becomes a savior figure (a messiah, some might even say) who fights to save his people from a spiritual death. Accordingly, the dragon then becomes a satanic figure bent on bringing about the spiritual corruption of Beowulf's kingdom. As such, taking the hoard to be a spiritual death of sorts greatly increases the significance of the final battle, as it lends the combat Christian undertones.
It's important to remember, of course, that many scholars believe the poem's Christian themes were at least partly added later on in an effort to make Beowulf less "pagan." As such, while it's true that Christian themes abound in the poem as it is presently read, we also have to take such ideas with a grain of salt.