Romans 15:16, 27

I am a special messenger from Christ Jesus to you Gentiles. I bring you the Good News so that I might present you as an acceptable offering to God, made holy by the Holy Spirit.

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.