Gandalf brings the dwarfs to Bilbo because he wants Biblo to join them on the treasure hunt. As a wizard, Gandalf knows that Bilbo is not an ordinary Hobbit. He tells the dwarfs that Biblo is a burglar, even though Bilbo has done no such thing. Gandalf knows that Bilbo has a special part to play in the adventure.
As it turns out, Bilbo does take to adventuring rather well. He is scared at first, but when he is captured by the trolls he tells them that he is a burglar, then changes him mind and tells them that he is a hobbit. They think he is a burra-hobbit. Gandalf rescues him by tricking the trolls into arguing until daybreak, when they turn to stone. This is the first time Bilbo realizes that he can be brave.
A secret entrance to the Lower Halls.