Are there any narrative detours in Beowulf? If yes, what purpose did it/they serve?

Although Beowulf is best known for its depiction of Beowulf and his struggle against Grendel and Grendel's mother, the poem contains several important digressions from that central theme.  These departures from the Beowulf-Grendel narrative serve to underscore the dynastic struggles that characterized Scandinavian and, later, Anglo-Saxon societies, as well as to shed light on Beowulf's role as, first, a leader among the Geats, and, second, as the Geats' king and principal protector.
Warfare among tribes and blood-feuds among families characterizes the world in which Beowulf lives and dies.  For example, in the first few lines of Beowulf, we read that Hrothgar, as a good king should in this society, distributes treasure to his retainers (his most trusted warriors) and builds a suitable hall for them to celebrate in,  the great mead-hall Heorot:

The hall soared high/ and its gables protected by horns; (the Heathobards' wrath and/ flames of hatred was still to come,/when armed struggle between sworn friends,/caused by hate would break out.) (ll. 83-86)

The poet makes this comment as an aside to the listeners and readers, which implies that they do not need further explanation to understand the reference, but it is the first of several digressions in which dynastic warfare is discussed.  In this case, the poet alludes to Heorot's ultimate fate when, in an attempt to make peace between the Danes and Heathobards, Hrothgar has his daughter Freawaru marry Ingeld, the prince of the Heathobards.  The marriage fails to prevent the two tribes from starting a disastrous war later on.
Another digression (see ll. 875-900) sets up the contrast between a good king, Sigemund, and a bad king, Heremod.  Sigemund, whom the poet describes as "by any measure the greatest/of all men, the protector of warriors" (ll. 898-899), becomes the standard by which good kings are measured.  Heremod, on the other hand, who is guilty of oppressing his subjects, is exiled from his tribe because "to his people, to all of his princes, he was a bitter problem" (ll. 905-906) and "crime overtook him" (l. 915).  Sigemund, who also kills a dragon during his kingship, is the pattern of a great warrior king in this society and is meant to represent Beowulf's goal as a leader.
The most important and lengthy series of digressions is introduced by what is known as the "Fight at Finnsburg."  This story,which recounts a battle between the Jutes and East Frisians, on one side, and the Danes, on the other.  Like the story of Freawaru and Ingeld, the princess of the Danes, Hildeburh, has been married to Finn, the Frisian king.  And like the outcome of the first story, this one ends badly when a battle occurs and Hildeburh's son and brother are killed by her husband's warriors.
In sum, then, the digressions may seem to be an intrusion, but most of these depict an integral part of this society's cultural inheritance--war, more war, and then a little more war.