A good example are Beowulf's last words to his cousin Wiglaf:
"You are the last of our family, Wiglaf. All the others fell when Fate decided they must. Now I must follow them."
Fate, of course, is not a specific entity but the events occurring after Beowulf and his ancestors made their decisions. Beowulf does not regret any part of his life, but wants it to be known that he fought and sacrificed for his people. Beowulf is ready to "fall" as is his "fate," and he entrusts Wiglaf both with the throne and the responsibility to make decisions as he did.
The rhetoric of this important text concerns the nature of heroism and bravery and in particular how this is examined through the character of Beowulf himself. He in many respects is a perfect hero because he considers the duty of a king is to sacrifice himself and his comforts for the benefits of his people, as opposed to building up his wealth and keeping himself safe, as Hrothgar does.
Beowulf's rhetoric can vary from situation to situation, which is part of what makes him such a fascinating character. For example, he is very respectful to the coast guard when he first lands in Denmark, even though he might have been tempted to be dismissive and arrogant. However, when dealing with the sarcastic and disrespectful Unferth, Beowulf shows that he is capable of sarcasm himself. Later, when he returns home and speaks to his king, he shows enormous respect to Hygelac. Beowulf is strongly praised by Hrothgar for being a young man who knows how to speak wisely.
Another argument in Beowulf is the role of Wryd or fate in a man's life. There are many things that man can control, but he can't control his fate; all he can do is be ready to fight the good fight or do what is most honorable in whatever circumstances he faces.
There is a strong rhetoric on bravery and the glory that can be won by being brave in combat. This is part of the reason why Beowulf wants to fight Grendel alone. If he defeats him alone, then there will be more glory for him. All of this is in keeping with the Greek / Roman past, where valor and glory were the highest pursuits of men.
I thought the question is asking about the rhetoric of the epic as a whole. In that regard, I think that the rhetoric of the epic is that the greatest glory that a man can win is to sacrifice for the benefit of his people. It seems to me that this is the general argument that this epic is making--that this is the source of Beowulf's greatness.
Given that Beowulf is a leader and renowned warrior, his rhetoric is important. His language must be elevated above that of the normal man. He, through his speeches, should be able to rally his warriors together to fight. He, therefore, is the epic motivational speaker.