After describing the demon possessed man in the Gadarene region, Mark shows how Jesus healed the man. He asked the guy's name. He said, "Legion," because there were many demons in the man. They begged Jesus not to send them to hell just yet. Jesus sent them to a herd of pigs who rushed headlong into the sea and drowned. The rest of the town told Jesus to leave, but not before Jesus made the newly healed man the missionary to that region.
Then, Jesus and company go back to the Jewish side of town and a synagogue ruler, Jairus, asks Jesus to heal his daughter. Jesus goes with him.
The presence of pigs shows readers that Jesus had taken his disciples to a place where Gentiles lived. Pigs were unclean to Jews and no Jewish town would keep them as livestock.
After Jesus had just weirded out his disciples by calming the storm, he made things even more weird by taking them to the uncleanest region he could think of. A Gentile town, with a herd of pigs, in a graveyard, with a demon-possessed man.
Perhaps Jesus was trying to teach them that without his presence, they are just as unclean if not more. Jesus came to save sinners. All kinds of sinners. He came to clean the unclean. And all people are hopeless until Jesus shows up and changes them in the inside, making them able to love and follow him.
It would take much longer for the disciples to understand that they were to spread the message to non-Jews, too. Peter had to be shown a vision of a sheet being lowered with unclean animals and God telling him to not call unclean what God has made clean, before he went to see Cornelius with Jesus's Gospel. But when that happened, he probably remembered this day in the uncleanest graveyard and told Mark to write about it.
When Jesus heals the man, the town not only shows that they are Gentiles, but also the kind of environmentalists that think pigs are more important than people. This precious man had been bound by Satan for so long, and the demons just kept growing and growing until nobody could control him. And all they can think about is that their pigs and income are all dead in the Galilean water.
The newly healed man wished to go with Jesus, but Jesus, having mercy on the town, told the man to share the Gospel with this town. Jesus's first missionary was this man. He got to be a missionary before the Apostles. God can to the same for you and your children too. If anybody you know it hopeless, he or she is not beyond our Lord's healing touch.