The answer to this question can be found in Chapter 20 of this text. This is the last time that Daniel and Joel actually meet in the text, as Joel explains that he has ran away from his father to join Daniel and to work with his insurgent group of rebels. The reason why he has decided to leave his father is that his father has arranged for Joel to be sent far away to study at a school. Note what he says to Daniel:
Then today he told me I was to go to Jerusalem--tomorrow morning--with a friend of his. He's arranged for me to study in a school there.
Joel is therefore moving to Jerusalem, and in this chapter Daniel persuades his friend to go and follow his father's wishes. Because of their faith in a new leader who will come, a better leader than Rosh, Daniel recognises that the Jews will need both fighters but also people who are educated and trained, like Joel. In a sense, he sacrifices his desires to be with his friend and to fight with him for the good of both Joel and the Jews. The act of Joel going to Jerusalem to study is therefore significant because it represents both Daniel's development and maturity in recognising the bigger picture of the resistance struggle and also his putting the interests of the Jews in front of his own interests.