Beowulf and Grendel: the fight
As suggested by the title, the passage tells about the fight between Beowulf and Grendel. It’s a struggle between good and evil.
Nor did the creature keep him waiting
but struck suddenly and started in; 
he grabbed and mauled a man on his bench, 
bit into his bone-lappings, bolted down his blood 
and gorged on him in lumps, leaving the body 
utterly lifeless, eaten up
hand and foot.

The function of these lines is to describe how Grendel tries to murder both warriors and civilians. He grabs and mauls men afterword bat their bone-lappings and bolts blood then gorges their body . 
The function of the alliterations both in the 2nd line (“s”)and in the 4th line the alliteration (“b”)are to give importance to the monster’s actions and speed .

Venturing closer, 
his talon was raised to attack Beowulf
where he lay on the bed, he was bearing in 
with open claw when the alert hero's
comeback and armlock forestalled him utterly. 

In these lines the first contact between Grendel and Beowulf. The monster opens his claw to attack Beowulf but immediately the hero armlockes him.
The alliterations in 2nd (“t”) and 4th (“h”) lines have the function to underline two keywords: attack and Hero.
The first keyword is opposite with line 5 where (“ck”) the verbs refer to hero, the second keyword.

The captain of evil discovered himself
in a handgrip harder than anything 
he had ever encountered in any man 
on the face of the earth. Every bone in his body
quailed and recoiled, but he could not escape.

The function of these lines is to describe behavior and reaction of the monster therefore he is trapped in a handgrip then his body quails and recoils.

In these lines there is a kenning ,a metaphorical name for something , in line 1 that’s why the captain of evil is used to describe the monster Grendel. What’s more , in line 2 (“h”) and 4(“b”) there are two alliterations: the first tries to underline the force of Beowulf besides the second describes the effects of the monster's body of its strength.

Time and again,
Beowulf’s warriors worked to defend
their lord's life, laying about them 
as best they could with their ancestral blades.

Warriors help Beowulf to defend himself that’s why the alliteration of the sound “w” focuses on warriors, their work and the weapon they use. They use ancestral blades ,i.e. object that is related to one existing at a later point in time.

Stalwart in action, they kept striking out
on every side, seeking to cut straight to the soul. When they joined the struggle
there was something they could not have known at the time,
that no blade on earth, no blacksmith's art 
could ever damage their demon opponent. 
He had conjured the harm from the cutting edge
of every weapon.

When warriors start to fight they notice every weapon hasn’t the harm because Grendel conjured them. No weapon is able to damage their demon opponent.
In the first lines the alliteration of sound “s” underlines the keyword soul that’s why the actions of warriors lead to tear the soul of the monster. 

The monster's whole body
was in pain: a tremendous wound 
appeared on his shoulder. Sinews split 
and the bone-lappings burst. Beowulf was granted 
the glory of winning; Grendel was driven 
under the fen-banks, fatally hurt,
to his desolate lair.

The last part of the fight where Grendel die and finally Beowulf is granted the glory of winning and he will be remembered in history.
Alliterations :”w” to underline winning ; ”s” to underline how the monster suffers.

The text is characterized by some alliterations of the letters “g”, “r”, which highlights the power of Grendel. Alliterations helps to memorize and to hand down the whole story.