Beowulf mentions the possible outcome of Grendel glutting himself on the Geats to emphasize the seriousness of the threat and the dire consequences if he fails in his mission to protect the Geats.It highlights Beowulf's determination to confront the threat head-on and his willingness to face the worst-case scenario, demonstrating his bravery and commitment to defending his people.By acknowledging this potential outcome, Beowulf shows his awareness of the danger and the high stakes involved in the battle against Grendel.