Returning to the places where the gospel had been preached and believers added to the Lord was for the purpose of strengthening (14:22; 15:32, 41; 16:5; 18:23) and establishing these new converts so they could stand firm in the faith on their own. By doing this, Paul could move on to other fields of ministry as led by the Spirit (16:6-10) and know that what he left behind was self-sustaining and multiplying without his being there. Paul developed this principle in Timothy so he would do the same as Paul had done (Philippians 2:19-20).
Striving for maturity in the body of Christ must be a primary objective of our ministry (see Ephesians 4:11-16). If this principle was being used by everyone on the mission field and in ministry, every unreached people group would be reached by now. Sadly, most are distracted by other agendas and we are missing effective opportunities to fulfill the Great Commission. Evangelism and establishing converts go hand-in-hand with the expansion of the gospel.