The climax of The Hobbit is the Battle of Five Armies, which begins as a confrontation between the armies of Elves, Humans and Dwarves arguing over the distribution of the treasure that lies in Erebor. However this conflict is interrupted by the arrival of an army of goblins and wolves, who are the enemies of elves, men and dwarves alike. They ally themselves against the goblins and wolves and attempt to trap them between two of the spurs of the mountain, hoping that their superior positions will make up for the superior numbers of the goblins and wolves. 

The elves take up the southern spur, and the dwarves and men the eastern one, bracketing the entrance to the mountain gate, which was guarded by Thorin's company. The southern spur, manned by the elves, happened to have an old watch post named Ravenhill, which was where the elvish king took up defense. Bilbo, who is mentioned as having been quite inconsequential to the battle's outcome, joined the elves at Ravenhill because he thought that escape might be most likely from that position, should it be necessary (as the western part of the southern spur faced away from the goblin and wolf armies, while the eastern spur was surrounded by them). 

Bilbo is also possibly stirred by the adventurous aspect of his heritage to stand by the elves, perhaps out of a sense of their nobility and innate goodness that would make dying in their service a better end than that of the more materialistic and conflicted men and dwarves.