There is nothing good that can come from this.
Actually, I disagree - in specific contexts. Most affairs have both positive and negative reactions within the person having the affair. The degree to which the person having the affair is having a positive or negative reaction depends on his/her particular circumstance.
In any marriage there are 3 'entities' : the relationship, an individual, and an individual. The 'relationship' is built by two individuals. If one individual is unhappy, the relationship suffers.
Often one, or both individuals have neglected the relationship. Sometimes Individual A expresses to the Individual B something that is missing in the relationship, and sometimes, Individual B just isn't willing to meet that 'missing' need of Individual A...with negative results on Individual A, which results in negative results on the relationship, and usually (if Ind B isn't blind) negative results for Individual B (and any kids involved).
While it is an individual responsibility to look after your own happiness, a major contribution to that happiness comes from the relationship.
...I'm sure most can see where this is going.
...affairs can in the short term, save marriages (I know of at least one that I'm quite certain lasted longer because of an affair, and heard of others)...because they can relieve stress, anxiety, and create an elevated/happier mood for the person having them.
I don't agree with affairs per se - I quite like the concept of marriage...and I also don't agree you should be miserable for years/decades in your marriage even after telling your partner what you need from the marriage, and them ignoring such.