If the discussion is about why people have affairs is focused on the needs of the individual, then it's pretty obvious how PM benefits the man. But to try and understand the woman's point of view in PM takes a little more work. Trying to maintain the order of headship, establishing the realm of trust and loyalty to a woman, bringing peace and love to her life, all revolves around what Christ does for the church and what we are to do as husbands. Sure, the old testament version says that a wife most obey. But the new testament version is based on the idea of one will obey because of love, not force.
For a man to benefit from PM, a woman has to know that she is loved, and loved deeper than any love she will ever find.