Beowulf has heard about the horror inflicted upon Herot in Hrothgar's kingdom by the monster Grendel. When he hears that every night, more and more of Hrothgar's men are killed (and eaten) in the mead hall, Beowulf rallies his men, and they travel to Hrothgar's land. Beowulf's choice to go to Hrothgar's kingdom shows that he is an epic warrior. He seeks out challenges, he is brave, and he has the physical strength to compete against a monster like Grendel. Beowulf also asks his men to go with him, and they agree, which shows the mutual respect and loyalty between them. 

When Beowulf arrives at Hrothgar's kingdom, he must ask permission of the king to fight Grendel. This shows Beowulf's respect for Hrothgar's authority and that he understands and follows the code of comitatus, a pact of loyalty between lords and their men. He asks Hrothgar to take care of his men if he (Beowulf) should fall in battle. He also gives Hrothgar a lengthy resumé of his heroic family and accomplishments. The battle against Grendel will add on to Beowulf's resumé, of course. This fight allows him to show off his courage and his larger-than-life, superhuman physical prowess. Beowulf is willing to put his life on the line to protect others. He goes out of his way and travels across the sea to voluntarily place himself in a life-threatening situation to save Hrothgar and his men from the wrath of Grendel.

Beowulf travels to Heorot because he has heard what Grendel has been doing -- how Grendel has been killing Hrothgar's men.  He goes to Heorot because he wants to test his skill and bravery against the monster and show Grendel how powerful the Geats are.

What this tells us about Beowulf is that he is motivated by a desire for glory.  He is coming not so much to save Hrothgar's people as to show his own prowess.

We can see this from what he says to Hrothgar when they meet.  Beowulf asks him for a favor and the favor is to allow him to go after Grendel.  He will not fight Grendel with weapons because Grendel does not use them.

So, once again, Beowulf is coming looking for glory (so it would be a favor to let him fight the monster), not to save Hrothgar's men (if he were there to save them, he'd kill Grendel whatever way he could).

Why does Beowulf go to Heorot?

Beowulf, a mighty warrior living in Geatland, learns that Grendel has been attacking Heorot, the great mead hall in Denmark built by Hrothgar. Beowulf therefore decides he must kill Grendel. He takes 14 of his best warriors and sails for Denmark.

Heorot, as a mead hall, is a symbol of civilization—of warmth, food, fellowship, and security—so Beowulf's decision to defend it places him on the side of all that is good, protecting human society from chaos and barbarism.

Hrothgar is glad to have Beowulf arrive. Hrothgar thinks that, in addition to friendship, Beowulf has come to repay him for helping Beowulf's father, Edgetho, avert a war with the Wulfings years ago. Hrothgar orders a great feast to celebrate the arrival of Beowulf and his men. That night, Grendel attacks, and Beowulf rips off his arm with this bare hands, for the soldiers' weapons have been bewitched by Grendel and are useless. This loss of an arm mortally wounds Grendel.