The mead hall is the center of Anglo-Saxon life both in the epic poem Beowulf and in actual Anglo-Saxon society. The mead hall is the center of social life and the center of defense and protection.
The mead hall is led by a gold-lord like Hrothgar. Men serve him and join him in battle and he in turn organizes protection for them.
No central government exists, so life is precarious. One mead hall can attack another and take over the mead hall. Defeated residents are often killed or exiled.
Grendel, then, in the poem, is every Anglo-Saxon's greatest nightmare. Grendel does in the poem what any other mead hall might do at virtually any time. Thus, though Grendel is a creation of imagination, of course, the threat he poses to warriors is actual.
Wealth, fame, and honor come through great deeds in battle. Bravery and courage are highly honored. Beowulf speaks repeatedly about his reputation and, in fact, his view of immortality centers on being remembered for great, heroic deeds in combat.