what is the purpose of the song about hildeburh the danish princess?

The minstrel sings a tragic tale that tells of the defeat of the ancient Danes. Wealhtheow gives a necklace that Beowulf's king Hygelac will wear when he falls. The section ends with "one beer drinker / ready and doomed [laying] down on bed." Things will become more and more difficult for the Danes and the Geats, leading to nothing but death. There have already been death-feasts (for Grendel and for the men dead by his hand); now there will be sleep-deaths (in this warrior sleeping and in the warriors before). Everything will eventually lead to ruin and death, despite the continuing parties.

Please take a look at the GradeSaver Link below as there is a little more to this answer,

http://www.gradesaver.com/beowulf/study-guide/section4/