The purpose was to carry out Paul's loving concern for the need of the poor saints in Jerusalem. At that time Paul was in Ephesus on his third ministry journey, busy with a heavy burden to carry out his ministry in Asia (1 Cor. 16:8-9) and in Macedonia and Achaia (1 Cor. 16:5-7; Acts 20:1-3). Nonetheless, he still had a burden to spare part of his time for the needy saints in Jerusalem. When he arrived in Corinth, he wrote his Epistle to the saints in Rome, expressing the purpose of his trip and begging them to pray for him concerning this purpose (Rom. 15:25-31). Although he was an apostle set apart by God for the Gentiles (Acts 22:21; Gal. 2:8), Paul was still concerned for the Lord's interest among the Jews. His primary concern was for the Body of Christ universally, not merely for his part of the New Testament ministry among the Gentiles.
Besides this, his purpose in going to Jerusalem at this juncture might also have been to fellowship with James and the other apostles and elders in Jerusalem regarding the Judaic influence on the church there. According to Paul's teachings in the Epistles to the Galatians and the Romans, the decision made by the conference of the apostles and elders in ch. 15 to resolve the problem concerning circumcision must not have been fully satisfactory to him. This undoubtedly troubled him because of his concern for God's New Testament economy, which is to build up the Body of Christ. James's word in Acts 21:20-22, after Paul had arrived in Jerusalem (Acts 21:17-18), and his proposal that Paul participate in the four Jewish believers' Nazarite vow (Acts 21:23-24) seem to confirm this view.