This is a good question. In some ways, I can see your point. It seems like Grendel is the fierce monster that needs to be opposed and once this is done, the story would end after his defeat. However, if you continue to read the story, there are two additional battles.
After the defeat of Grendel, there is another foe that Beowulf needs to face. In fact, he almost loses his life in the process. He needs to fight the mother of Grendel, who is now furious and wanting revenge. The fight between Grendel's mother and Beowulf is epic but in the end Beowulf is victorious.
After this there is a time of peace. Beowulf goes back home to Sweden to the kingdom of Geats of which he is king. But after 50 years there is a final challenge. Beowulf needs to protect his own people from a fierce dragon, who is on a rampage on account of stolen treasure. Beowulf once again succeeds, but in the process is fatally wounded.