Why did Paul go to Jerusalem instead of straight to Rome?
Paul went to Jerusalem first to minister to the believing Jews there and to deliver the contribution from Macedonia and Achaia for the poor saints. Although he intended to stop at Rome and go on to Spain, he was determined to go to Jerusalem first, not knowing he would later reach Rome as a prisoner.
Les explains that Paul had been constantly looking for the day he would not only stop at Rome but also go on west to Spain, and he wrote that he would come to them if first filled with their company. But first and foremost, he went to Jerusalem to minister unto the saints, the believing Jews there. Les points out that Paul had no idea he would end up going to Rome by the prisoner route. Paul had been laboring in western Turkey and then in Greece, Achaia, and Macedonia. Those new believers were intensely poor, yet they gave so Paul could take the offering to the poor saints in Jerusalem who were Jews.
But now I go unto Jerusalem to minister unto the saints.