It isn’t really a matter of Beowulf having to battle Grendel with his bare hands but that he chooses to. Beowulf is clearly a hero in this epic poem. One trait most heroes display is having strength. Another trait many archetypal heroes display is a certain hubris, or arrogance. This situation shows both of these traits in Beowulf. He chooses to battle Grendel using no weapons to prove he is strong. He also chooses to do so in order to remain on the same level as Grendel; Grendel uses no weapons, so Beowulf doesn’t either. It may be that arrogance pushes him to "prove" himself and his strength in this battle or, since he has proven himself enough already to be a champion, he may be doing to gain the glory and honor warriors strove for to make themselves worthy of respect and wealth accumulating gifts.