Read Romans 15:27 Using Other Translations
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.
For they were pleased to do it, and indeed they owe it to them. For if the Gentiles have come to share in their spiritual blessings, they ought also to be of service to them in material blessings.
They were glad to do this because they feel they owe a real debt to them. Since the Gentiles received the spiritual blessings of the Good News from the believers in Jerusalem, they feel the least they can do in return is to help them financially.