The songs of the bard are important in the beginning of Beowulf because they ground the epic historically to the period from which it was written. During the time of Beowulf, a common written language did not exist. Instead of communicating through written word, and instead of writing down the tales in order to pass them on, the tales were sung by bards (or more commonly, as used by the Anglo-Saxons, scops). These men sang songs about the heroes and myths of their culture.
This singing of songs allowed each scop to interpret the tale and manipulate it as their own. There is no telling how many different tales of Beowulf actually exist (even the formal, written translations of Beowulf are numerous).
This inclusion of the scop also explains the use of "I" within the text. The narrator (or scop) was defining (for his or her audience) the telling of the tale is an individual one.