This epic tale, like others of its ilk, reveals the importance of myths and legends in creating an identity for a nation and a sense of history and shared past that is so crucial to carving out a name and a sense of nationhood. Clearly, key themes of this epic tale are the ancestral heritage of the Geats and also the reputation of key heroes in their lineage. Family history is emphasised from the outset, as the beginning of the poem makes clear. The reader is introduced to a world where every male character is known by their father's name and who your family are is a massively important issue. Warriors take massive pride in ancestors who have attained the status of heroes, and this encourages them in turn to live up to the reputation of their ancestors. Note for example the way that Shield Sheafson is referred to as this epic opens:
So. The Spear-Danes in days gone by
and the kings who ruled them had courage and greatness...
There was Shield Sheafson, scourge of many tribes,
a wrecker of mead-benches, rampaging among foes...
A foundling to start with, he would flourish later on...
In the end each clan on the outlying coasts
beyond the whale-road had to yield to him
and begin to pay tribute. That was one good king.
The importance of this myth and other myths and legends in general is therefore that it creates a number of heroes that a people can look back to and take pride in. These heroes possess the characteristics that define what it is to be a "good" person in that culture. Thus it is that Shield Sheafson comes to be, along with Beowulf, a hero for the early Anglo-Saxons to look back on and to help define them as a people. The importance of an epic in helping to form an identity is thus emphasised.
Myths and legends impact a civilization in many ways.
Myths form the basis on what that certain civilization will base their laws and mores on. For example, in Ancient Greece, well-known myths about the Olympian gods Zeus and the like made people aware that if they did something that will upset the lighting god, they will receive punishment, or if they have lived their lives accordingly, they will be cast by Hades into a better level in the underworld.
Civilizations are also defined by their architecture, in which they built these structures to honor mythical beings. A good example would the temples left in Ancient Greece, or the monuments/altars Mayans left. These were used to honor their gods.
Hope this explanation helps.