The Gadarene man who had been demon-possessed would never forget the encounter he had with Jesus of Nazareth. It was the defining moment of his life.
In this sermon, we see the messenger sent: "Go home to your friends." The redeemed man has a sphere of service immediately to hand.
Then consider the story told: it is a story of grace, a disciple's story, personal, truthful, grateful, humble, joyful and earnest.
Finally, we observe the reaction sketched. In the short term, shock: people were amazed, perhaps troubled and disturbed. But in the longer term (see Mark 7:31 - 8:10) perhaps this man's efforts broke the ground for the return of Christ.