It is not clear how Onesimus was of assistance to Paul. Perhaps he served to run messages and bring food. Maybe he was being trained to become a minister of the Word.1 If Paul was old, then he would have appreciated the assistance that can be provided by a younger man.
13 J'aurais désiré le retenir auprès de moi, pour qu'il me serve à ta place, pendant que je suis dans les chaînes pour l'Evangile.