The golden drinking horn proved useful in the cave, but it did not look like Beowulf (from the 2007 movie) knew it would. Why did he bring it, especially since his sole purpose to visit that cave was to kill the witch?
The golden horn was a gift/talisman bestowed upon Beowulf by the old King. The old man attributed a great deal of importance to this item, and Beowulf took it with him, partly to use in the cave and partly to display to the 'witch' his alignment with the King.
However, the horn itself was a gift given to the old king by the witch in return for a son (Grendel), and by ensuring that it was now in Beowulf's possession a gesture similar to 'passing the torch' has been made. Now it is Beowulf's turn to give the witch what she wants (a son, revealed upon the beach at the end of the film) and the horn is the symbol of his illicit pact with her.