Question

Why use a Epithet or kennings in Beowulf?

Answer 1

Such literary devices helped the (usually illiterate) storytellers (bards) remember the lines so they could tell the whole story.

Read about the oral tradition in storytelling in many of these search results:
http://www.google.com/search?q=old+stories+told+oral+tradition&oq=old+stories+told+oral+tradition&aqs=chrome..69i57.5947j0&sourceid=chrome&ie=UTF-8