Final answer:
Beowulf mentions the possibility of Grendel glutting himself on the Geats to emphasize the severity of the threat and the urgency to defeat the monster.
Step-by-step explanation:
In line 178 of Beowulf, the titular character describes the first thing that will happen if Grendel wins: he will glut himself on the Geats. Although Beowulf has come to protect the Geats, he mentions this possible outcome to emphasize the severity of the threat posed by Grendel. By highlighting the potential consequences of Grendel's victory, Beowulf reinforces the urgency for his mission and the need to defeat the monster.