After warning Gaius not to follow the ungodly example of Diotrephes, a proud and controlling church leader (3 John 1:9-11), John points to Demetrius as a reliable role model in ministry. That man was transparent to the church; he had no hidden agenda, he was a servant of Jesus Christ whose ministry was endorsed by the apostles.
Such men are a blessing to the church, not only by their fine godly service but because they inspire others to use their gifts in a way which pleases the Lord.