The benefits are huge if it is something that pleases both partners. For my wife the arousal she gets from having sex with another man absolutely floods into out own relationship. If she and I have sex within a short time of her seeing her regular partner, her vagina is more supple and she lubricates far more. She is more eager, much more adventurous, and always in a great mood after.
Early in our marriage, when she decided it needed to be me and only me, the arguments were more frequent and our sex life was getting a bit vanilla which did not suit either of us. When we talked about her going back to having a second male for sex in her life, her whole being changed. We talk more openly about everything, never have to be concerned about anything being hidden, and talk about things that would make many people’s eyes pop. She tells me his fantasies and tells me her own, whether he took a long time or was quick, where they met and what happened during their meeting. These conversations can be held anywhere. In bed. In the car. Having dinner. Even doing housework.
There are so many good sides to it for us, but that does not mean it is for everyone.