A small detail of Jesus' actions on earth shows us how the natural and supernatural worlds overlap.
Though Mark and Luke focus on only one of two the two possessed men encountered in this scene, Matthew 8:28-37, Mark 5:1-20, and Luke 8:27-38 all tell the story of a showdown between Jesus and a bunch of demons. Stepping on to the shore of the Galilee with his disciples, Jesus is met by two men. Their possession by evil spirits has led at least one of the men to cut himself, live outdoors among tombs, and given him enough strength to break the restraints put on him by members of the nearby town. That obsession with death and self-loathing follows demonic possession should come as no surprise. In John 10:10, Jesus tells us that The thief comes only to steal and kill and destroy; I have come that they may have life, and have it to the full.
The demons recognize Jesus immediately as the Son of God and the men fall before him. When Jesus asks their name, they say that their name is Legion, being a multitude of demons. As Jesus casts them out, he gives them permission to enter a herd of about two thousand pigs. These pigs rush into the sea of Galilee and drown.
The Bible isn't clear as to why the demons want to enter the pigs, though the account in Matthew suggests they want to avoid their end of "torment" for as long as possible. Revelation tells us that the end of all fallen angels is the "lake of fire". Demons seem to be confined on earth until their final judgement. They must need a body to inhabit to work through and there's other language in the Bible that refers to territorial assignments.
By allowing them to enter the pigs, Jesus sent a message to the herd's owners, who are either culturally Greek gentiles or recalcitrant Jews, and the nearby town: these men, abandoned and hated by their community, are incredibly valuable to God. It also speaks to Jesus' complete authority. He has power over demons, power over creation, and authority over our lives. Mark tells us Jesus commissions one of the men, sending him joyfully into the ring of Greek towns in the area to tell about God's goodness and the event. The next time Jesus returns to the area, there are crowds of people waiting for him.