The purpose of pre and post nuptial agreements is to delineate how property is owned and define are expenses are to be paid from the date of the agreement forward. By law, all property within a marriage belongs to both parties and all debts are owed in common regardless of the name on the asset or debt. A prenuptial or postnuptial agreement can alter that by defining the intent of the parties of how assets are to be owned and debts to be paid.