The answer to this question can be found in Chapter 16, in which the latest orders of Rosh are given to Daniel. In order to carry these orders out, he needs Joel's help, and so he goes to the city to speak to his friend. Rosh has told him that King Herod is hosting a special banquet to welcome a special legation from Rome, and all of the rich and powerful in Capernaum will be present at this banquet. Rosh wants to know the names of those who will attend this banquet. Note what Joel says in response to this request:
I can see why. It would give us a good idea which men would be against us. Any Jew who would eat at the tetrach's table--
Joel therefore assumes that Rosh wants to know who will attend the banquet in order to discover which members of Capernaum society can be depended upon to support the resistance and which are in league with Rome. Of course, as Joel and Daniel discover later on in the story, the real reason is that Rosh plans to use the banquet as an opportunity for robbery, as he will know which rich residents will be away from their homes. This is actually a significant event in the novel in terms of charting Daniel's growing disillusionment with Rosh and his realisation that Rosh is not the resistance hero he thinks he is, but rather nothing more than an opportunistic individual who seeks to personally profit himself alone and is not interested in the liberation of the Jews.