The rationale is that handling the aircraft should be well within their capabilities
The man must be a religious type as he has FAITH in his first officer. The rationale is flawed. If the bloke in the right hand seat is experienced and the captain knows from previously flying with him that he can handle the situation efficiently, then all well and good. On the other had if you have an MPL as second in command with no skills except button pushing, then best fly it yourself. Horses for courses.