One of the things that helped Chaucer's Canterbury Tales to stand out from much of the literature of the time was in fact these changes in perspective and personality from the various narrators or characters. By giving them different stylistic touches, different vocabularies and different ways of looking at the various tales, Chaucer did a good job of actually trying to find ways to present perspectives that were not often represented in literature at the time, those of more common perspectives like the cook.
Chaucer's motive was most likely just his own will to be a dynamic story teller. Chaucer is frequently regarded as the Father of English literature. The Canterbury Tales were presented in a brand new format never before seen in English literature. Boccaccio's Decameron does however resemble The Canterbury Tales, as they both have a frame that connects their multiple stories, but Chaucer's work is much more full and polished. In Chaucer's time, the norm was to only base your stories after stories previously created, Geoffrey Chaucer had no problem breaking free of that norm, as evidenced through the tales. He encompasses nearly every aspect of medieval England, starting with nobility, see the Knight and the Squire, followed by the clergy, see the Monk and the Prioress, and finally the middle class, see the Wife of Bath, the Miller, and all the rest.
Chaucer was well positioned and well favored in his time, and his home was strategically located in an area where he would have gotten a very in depth look at the lives of all these types of people. His decision to take on the tone of each character, and his reflection of their own personality's within the actual stories was ingenious.
His transitions between the individual stories are also very interesting, every prologue sets up a certain dynamic among the pilgrims, allowing individual relationships to form, making the stories themselves more interesting. Chaucer could just have easily decided to do an anthology of stories about these characters that had no connection whatsoever to each other, but instead he builds the reader's interest by exposing character flaws and poking fun of the corruption of the time.