“When [Apollos] arrived, he assisted greatly those who had believed by grace, for he refuted the Jews vigorously in public debate, demonstrating from the scriptures that the Christ was Jesus.”
In Achaia Apollos defended the gospel among the Jews. This greatly helped the saints, because he aided them in their mission and strengthened their faith. “For” identifies his preaching as the means of his assistance.
Evangelism benefits the whole church. It is central to her identity and edifies all in every way. The best one can do for the church is to teach others how to be saved.