Every time I pray, he says, I remember you in my prayer. Remembering you, I thank my God for you.
By saying that he thanks God for Philemon, he lets him understand that he is pleased with him. He acknowledges him as perfected in Christianity, and thereby gladdens him and encourages him toward still greater good.
This is not foreign to the purpose of the epistle. Yet by such thanksgiving he also raises Philemon's spirit with his own before the face of God.
He teaches him to see in God the source of his perfections, and to humble himself all the more in awareness of them, rather than think highly of himself.
He should strive in every way, like the Apostle, to appear thankful before God. This too is not far from the purpose of the epistle.
For one must give thanks not only by word, but much more by works pleasing to God. One such work the Apostle will immediately set before him.