The disciples were "saddened" for they understood Jesus' prediction: that He would be "handed over". The death and resurrection of Jesus was and is the very reason for the existence of the Christian community of disciples, then and now. For the community of Matthew, living during a time of severe persecution, this was a redeeming factor. The calm assurance with which Jesus faced His hour was an encouragement to witness one's faith when faced with certain death at the hands of the authorities.
Further, as God's children, they did not have any obligation towards paying taxes to maintain "Jupiter's" Temple in Rome. But Matthew uses his ecumenical diplomacy and pastoral good sense to instruct his community to pay their dues in order to avoid further problems in their difficult circumstances.
Jesus Himself had complied with the rules, though as Son, He was exempt from contributing to the Temple Tax, as the Temple was His Father's house. Scandal is to be avoided for the sake of peace, harmony and reconciliation. The following of Christ tolerates no half-measures or short-cuts to holiness. To be 'salt of the earth' and 'light of the world' the disciple of Christ would always need to take the 'road less-travelled.'
Your Word, Lord is a lamp to my feet and a light for my path.