John has just been speaking about the Christian privilege of prayer; and now he goes on to single out for special attention the prayer of intercession for the brother who needs praying for. It is very significant that, when John speaks about one kind of prayer, it is not prayer for ourselves; it is prayer for others. Prayer must never be selfish;, it must never be concentrated entirely upon our own selves and our own problems and our own needs. It must be an outgoing activity. As Westcott put it: "The end of prayer is the perfection of the whole Christian body." —Wm. Barclay
This from his commentary on 1 Jn 5.
And he also says this, which we need to hear, since our church bulletins are full of physical needs, but almost nothing of spiritual:
(i) We naturally pray for those who are ill, and we should just as naturally pray for those who are straying away from God. It should be just as natural to pray for the cure of the soul as it is to pray for the cure of the body. It may be that there is nothing greater that we can do for the man who is straying away and who is in peril of making shipwreck of his life than to commit him to the grace of God.