When Grendel tries to attack the place, it is seen as the attack of chaos aimed at structure and order. "Then a powerful demon, a prowler in the dark, / nursed a hard grievance" (86-87). Grendel is not an intelligent enemy but he is definitely powerful. His immense power turns him into a dangerous force since reason doesn't reside inside him. The hall was a symbol of civilization as the poet informs us: "inside Heorot / there was nothing but friendship" (1017-1018). Thus hall has immense symbolic value in the book and is aptly described as the "greatest house / in the world" (145-146). Beowulf is perfectly aware of the importance of Heorot. He knows that by saving the place, and defeating Grendel, he could actually be presented with the greatest house on earth. In a passage, he acknowledges the worth and value of this place:

The men hurried forward, pressed on together to where they could plainly see the timbered hall, splendid and gold-bright; that hall was by far the most famous to earth-dwellers under the skies, where the mighty king was waiting. Its light shone out over many lands. (306-311)]

Beowulf's worth is raised by the fact that he could purify the hall of Grendel. He is seen as a deliverer much in the same way as Jesus. This is evident from the lines Hrothgar's wife speaks when Beowulf gains victory. She sees Beowulf as "a deliverer she could believe in...to ease their afflictions" (626-628). Grendel is seen as the "captain...

"His actions challenge the identity that society has established for itself by transforming Heorot, the human equivalent of God's act of Creation, back into the negative state of pre-creation: the hall stands just as 'empty and useless' (and is described using the same words, idel ond unnyt [145b and 413a)) as chaos is in the Old English Genesis, with the same sinnibte 'eternal night' (161b) hanging overhead." (Neville: 74)
The hall is thus the most important object in the poem. It is the arch-image without which the poem wouldn't have existed or would certainly have lost its structural and narrative richness, vibrancy and beauty.

References

Alvin a. Lee, "Heorot and the Guest-Hall of Eden: Symbolic Metaphor and the Design of Beowulf," in the Guest-Hall of Eden: Four Essays on the Design of Old English Poetry, Yale University Press, 1972, pp. 171-223.

Jennifer Neville, Representations of the Natural World in Old English Poetry (Cambridge, England: Cambridge University Press, 1999)

Thomas Greene. The Norms of Epic," Comparative Literature 13 (1961), 193-207

Halverson, John."The World of Beowulf."ELH 36:4 (1969): 593-608.Rpt. In Readings on Beowulf. San Diego: Greenhaven Press, 1998

Beowulf: A New Verse Translation. Trans. Seamus Heaney. 1st ed.New York: Farrar, Straus and Giroux, 2000.