St. Thomas Becket was the Archbishop of Canterbury who was murdered in 1170 in the cathedral. He was later canonized as a saint, and his tomb became a popular pilgrimage destination. Pilgrims believed that visiting Becket's tomb could bring them spiritual healing and forgiveness.
The Canterbury Tales takes place during a period when pilgrimage was an important part of medieval life. This is why Chaucer chose to use a pilgrimage as the framework for his story.