Reasons for writing the Gospel
Mark doesn't explicitly state why he is writing the Gospel but there are themes running through his writing that suggest why he wrote the Gospel:
1) the Identity of Jesus
Mark's Gospel clearly shows us that Jesus had humanity, even though he was the son of God. He experiences human emotions such as anger and fear of death. Mark shows us that Jesus needs the support of the disciples and that he thought that God had deserted him.
However, Mark also shows that Jesus had great authority (he can perform miracles, heal people and control the forces of nature. Ultimately, he also overcomes death by being resurrected.
HUMANITY: Some Christians think that Mark's presentation of Jesus was intended to support Christians in Rome during a time of crisis. When they were afraid or angry, they would feel that Jesus understood.