Jesus usually told people not to talk about His miracles to help them. Why not this time? Because Jesus was in a Gentile region. The people did not have false expectations about the Promised Savior. They did not have any expectations. So the Gentiles needed to hear about Jesus to know what He had done for the man who had previously been demon possessed. Reports from the early Christian Church tells us that this region called the Decapolis or Ten Cities was very receptive to the Christian faith.
We are living in a time when fewer people know about Jesus. They need to hear so that can know who Jesus is, what He has done and what He wants to do for them. Tell everyone because whether they know it or not, everyone needs Jesus.