Answer:
The storyteller gives Grendel a mother for two reasons: to bring a certain humanity to Grendel and to highlight how unpredictable dangerous things are.
Explanation:
In Beowulf we can see that Grendel has a mother. This was added to the story as a way of showing that Grendel is not only showy, but that he had a certain humanity, being able to establish family relationships with the being who generated him. We can see that the relationship between him and his mother is beneficial, since she feels the need to avenge Grendel's death.
grendel's mother also represents the unpredictable, since in killing Grendel, Beowulf believed he had ended the danger and did not expect Grendel to have a mother who would seek revenge.