In this chapter and the next, Paul talks about how the church should take care of itself. People are behaving in ways that are wrong even in the eyes of non-Christians. This makes the church and belief in Jesus look like a bad thing. He wants them to address the problems in the church rather than criticize those outside of it.
When we lovingly help someone who is a Christian live life in a better way, we help the person and the church. The person gets closer to God, and the church gains a reputation for helping people live better lives.