Here's why:
* Kenning is a poetic device used in Old English and Old Norse poetry where a noun is replaced with a descriptive phrase. It essentially acts as a synonym or figurative name for the noun.
* In this case, "fell spoil" is a kenning for treasure. "Fell" means fierce or deadly, referring to the enemies Beowulf has slain, and "spoil" refers to the spoils of war.
* "Faring" adds an extra layer, suggesting Beowulf is journeying with his treasure.
Therefore, the entire phrase is a kenning for a warrior returning from a victory with his riches.
While alliteration (the repetition of the "f" sound) is also present, it's secondary to the use of kenning.