The Prologue provides the frame work for the individual tales being told throughout the narrative. In this General Prologue, Chaucer outlines all the characters and briefly describes them. He also sets up the reason for telling tales. As the pilgrims begin their journey, they tell stories to past the time. They make it into a competition, and are each supposed to tell four tales.
Much like the first act of a play, the prologue provides expositional information that is necessary to telling the tales. Without the Prologue, the tales aren't connected. The setting, characters, and reason behind the narrative are all established in the Prologue.