A prenup or premarital agreement is a contract between you and your perspective spouse. Why would you want to do this? You would want to do this if you had been married before and you had assets and children from a prior relationship and you wanted to protect those assets and you also wanted to provide for your children.
Another reason why you would want to do a premarital agreement or same thing as a prenup is if there is going to be a disparity in income or earning ability between the parties. Perhaps, you want to protect a family inheritance. You want to protect a family business. You want to clearly delineate your rights and responsibilities going into a marriage relationship.