tell the reason why the author begin with that opening scene

The opening scene is significant because it supplies us with important information about our setting and situates the story in a particular time and place. The narrator allows us to believe that he's telling a story, but beginning with line twenty he reveals that he is an actual person in the story by moving into a first person narrative (I), which nine lines later switches to the first person plural (we).

Later, he tells is that the story is being written some time after their original happenings, and that the stories themselves are told from memory. In essence, the stories he tells are shadowed by his own opinion and perception.

The narrator is also careful to categorize the members of the pilgrimage into group........... and each class of society is represented.

The Knight and Squire represent the military estate.

The Prioress (and her nun and three priests) represent the clergy. the Monk, the Friar, and the Parson.

Franklin and the Plowman represent the laity, and so on....

For the rest of your question, are you referring to the end of the opening scene or the conclusion of Canterbury Tales?

the significance of the closing scene