Question

Why might the poet have chosen to provide such as detailed description of the setting of Beowulf's battle with Grendel's mother. How does this description influence the reader's impression of the battle

Answer 1

It happens in the depths of the lake that Beowulf dives into and then in the vault that Grendel's mother takes him to. It is clear that both the depths of the lake and the vault are described as very dangerous places that threaten Beowulf's life. 

Answer 2

the poet establish  that that the coming battle is not just a battle between a hero and a monster  and the detailed description  chosen these themes stated, the most universal is good verses evil , so this description influence the reader's impression of the battle.