Geoffrey Chaucer inserts himself into his masterpiece, The Canterbury Tales, as the narrator. In general, Chaucer doesn't have much of an active function in the poem itself, and his primary purpose is to observe and describe the other characters and report their tales (Chaucer does tell a few tales of his own, but the first is interrupted and criticized heavily, so it's clear the author isn't afraid of making fun of his status as a poet).
By inserting himself into his own poem, Chaucer subtly asserts that he directly observed everything he reports, a claim that lends extra credence to his narrative. Additionally, by separating himself from the characters during most of the tale-telling, Chaucer is also able to separate himself from their lewd and inappropriate stories. Many of the tales are bawdy even by contemporary standards, and so they likely would have offended audiences in the Middle Ages. By positioning himself as a passive observer of the action, Chaucer skillfully avoids blame or criticism for the inappropriate aspects of his poetry. We can almost imagine Chaucer speaking in his own defense, saying, "I didn't come up with this stuff! I'm just telling you what I heard!"