Why was it fitting for the Gentile churches to give money to the Jewish believers in Jerusalem?
It was fitting because the Gentile believers were debtors to the Jewish saints in Jerusalem. Their contribution to the poor believers there was not out of place but was appropriate and as God intended. Scripture presents it as a proper debt owed by the Gentiles to the Jews, as Romans 15 explains.
Les explains that at that time there were no mission boards, no foreign missionaries to support, and no large church plants. Local churches met in homes and did not pay salaries or have such expenses, so their responsibility was the poor believers in Jerusalem. The one going to Jerusalem was going to minister to the saints, not to preach, since he had no ministry among the Jews; his ministry was financial help. Macedonia and Achaia made a contribution for the poor saints at Jerusalem. They were debtors, and the Greeks were not out in left field by giving offerings to these Jews; it was apropos and as God intended. The explanation begins in verse 28: “For if the Gentiles have been…”
It hath pleased them verily; and their debtors they are. For if the Gentiles have been made partakers of their spiritual things, their duty is also to minister unto them in carnal things.