Why do the dwarves come to Bilbo's home?

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.