One may have an S.S.O. to fulfill their some of their non-sexual emotional needs, while leaving them free to have sex with whomever else and however many others they want.
It is not unusual for one of the persons to be married or in a relationship. It is best for an S.S.O. to be attractive, because they are seen together all the time, but they shouldn't be sexually attracted to each other (at least not that much).