What is the main purpose of the journey that Bilbo takes with the dwarves? What convinces him to go? How does the purpose change for Bilbo, for the dwarves?

Bilbo is to play the burlar of the group trying to recover the treasure from the dragon.

Gandalf gives hom no choice but to accompany the dwarves.... they don't even ask; it is simply assumed, and off Bilbo goes!

The purpose changes when after the treasure is recovered, the group finds itself settling the differences between the dwarves, elves, and men....... even going so far as to participating in war against the goblins and Wargs.