Answer:
This establishes Beowulf's reputation.
It is important that Hrothgar has already heard of Beowulf because it establishes Beowulf's reputation as a strong and capable warrior. This reputation precedes him and is likely to have contributed to Hrothgar's decision to seek out Beowulf's help in defeating the monster Grendel. By establishing Beowulf's reputation, the story also hints at the challenges that Beowulf has already overcome and the successes he has already achieved, which sets the stage for the battles to come.