"Again the one who looked like a man touched me and gave me strength. 'Do not be afraid, O man highly esteemed,' he said. 'Peace! Be strong now; be strong.' When he spoke to me, I was strengthened and said, 'Speak, my lord, since you have given me strength.'" NIV translation
So, why did God send a vision of a man to Daniel? We see that the man comforted Daniel and gave him strength for the rest of the vision. This shows us man's role in the spiritual battles is to build each other up in the faith. Those who are stronger in their walk should share their faith stories to encourage those who are less mature in their faith.
"So he said, 'Do you know why I have come to you? Soon I will return to fight against the prince of Persia, and when I go, the prince of Greece will come; but first I will tell you what is written in the Book of Truth. (No one supports me against them except Michael, your prince.)'" NIV translation
The angel in the form of man tells Daniel that he is going to go back to Persia where he was going to usher in the defeat of the Media-Persian empire at the hands of the Greeks. The angel explained that he was going to show Daniel what would happen to the Jews under the Greeks. He tells Daniel that this is strictly an act of God as he says that "no one supports me against them". He also gives Daniel comfort by reminding him that the archangel Michael, who looks out for the Jews, is in the battle.