Well there aren't really any serious benefits of having extramarital affairs apart from maybe the joy and excitement of doing whatever it is that you're doing. Well I could say that the benefits depend on just why exactly you're cheating in the first place, and I think that would be a more accurate way to put it.
If you're married to a woman that's constantly nagging and doesn't give you peace then most likely what you're gaining from your extramarital affair is peace of mind, the likes of which you don't get when you're with your wife. Yes cheating is wrong but it most certainly has its benefits. If you're with a man that doesn't give you attention and doesn't know how to make love to you then what you'd gain from your extramarital affair is what your husband doesn't give you.
The truth is that in most cases, what you're gaining from your affairs are what you were missing in your marriage. For the most part, people won't cheat if they have everything they want in their partner and if they can't be content with what they have, they step out of their marriage to get it. So then, to put it simply the benefits of having extramarital affairs are the things you gain from the affair that you don't get from your marriage.
I hope this helps.