The goal of any cheater is to (hold onto) all that is "good" in their primary relationship while addressing their other "needs" on the side.
Essentially they're looking to fill whatever gap they perceive in their relationship. This varies from person to person whether it be something as simple as being bored, missing passion/romance, feeling neglected or taken for granted by their mate, sexually frustrated, or unhappy with aspects of how they're been treated by their mate.
Very often people ask: "If she or he is so unhappy why didn't they end their relationship?" What the asker fails to understand is cheaters are NOT looking to breakup or go through a divorce.
Their goal is not to sacrifice one relationship of another. They are trying to put all the pieces in place to have what they feel will make (them) happy. Cheaters don't expect to be caught!