The use of alliteration in this passage from the Anglo-Saxon poem Beowulf serves to add emphasis to the battle. It does so by creating a kind of punch as the alliterative words are read, something like a great "thunk" each time the fighters crash against against the walls and benches of the mead hall.
Note that the alliterative words in this passage are not overwhelming as they are in other passages but are spaced fairly consistently.
And wild. Heorot trembled, wonderfully built to withstand the blows, the struggling great bodies beating at its beautiful walls; shaped and fastened with iron, inside and out, artfully worked, the building stood firm. Its benches rattled, fell to the floor, gold-covered boards grating as Grendel and Beowulf battled across them.
Wild, wonderfully built, withstand...rest. Built, blows, bodies beating, beautiful....rest. Iron, inside...quick hits against something in the room. Fastened, artfully, firm, fell, floor...the battle is in full force. Gold-covered, grating, Grendel...and the fighting continues. We know that this pair is evenly matched in strength, and these examples of alliteration serve to punctuate the struggle between Beowulf and Grendel as they throw one another around the room.
We know this battle is mighty because the narrator tells us both in the quote above and in this more complete description that it is a miracle that Heorot, the greatest (and presumably best-built) mead hall in the land, is still standing when the battle is over.
Then it was a great wonder that the wine-hall withstood the war-fighters, that it did not fall to the ground.
Again, the use of alliteration serves as a verbal indication of the crashing and blows the two fighters were making in the mead hall that fateful night.