Plain and simple: If there were no monsters, there would be no opportunities for heroism.
But that's not the only reason there are monsters lurking in every corner of this poem.
More on this motif—next.
Besides Grendel, Grendel's mother, and the dragon, in this poem we're also treated to
descriptions of troll nests and beasts and horrific creatures from the sea.
Like I said—monsters in every corner.
That's what makes them a motif.
But what's the meaning of this motif?
Well, first of all, you should know that in medieval times, the word monster referred
to birth defects, which were considered to be a sign of sin, or a warning of bad things
to come.
So the monsters in this poem could represent the ever-present threats to the various nations
in this poem—the threat of chaos, lurking on the edge of civilization; the threat of
evil and destruction lurking on the edge of humanity and community.
But perhaps most important of all is the role these monsters play in supporting this poem's
theme of heroism.
The monsters give Beowulf an opportunity to prove himself heroic in battle.
Not only that, but their greed, inhumanity, and pure evil help highlight Beowulf's generosity,
humanity, and almost messianic goodness.
In other words, their monstrosity reinforces his heroic qualities.