Prenuptial agreements are often used in two types of situations: when a couple want to establish what will happen in the event of divorce, or when a couple is concerned with property rights upon the death of one party. In either event, a prenup should be carefully drafted by a professional to withstand future scrutiny in court.
Prenuptial agreements are best known for setting forth the rights that each party will have to property and support in the event of a future divorce. While this is seen by some as unromantic, it can be better thought of as an administrative procedure before a marriage, much like obtaining a marriage license. A prenuptial agreement doesn't mean that the soon-to-be-newlyweds expect the worst, but rather that they want to minimize the cost and acrimony that can come if things don't work out between them. In this way, a prenup can be used almost as an insurance policy - you hope it's never needed, but if it is, it can save you considerable expense in the event of divorce by minimizing areas of dispute.
Prenuptial agreements are also often used by couples when one party anticipates a large future inheritance of family assets or when two people who have already raised their families plan to marry one another. A prenuptial agreement is used, in combination with wills and other estate planning documents, to set forth what each person will and won't be entitled to when the other dies. In these situations, a prenup works to establish that the parties are on "on the same page," so to speak, bringing comfort to family members. Most importantly, a well-drafted prenup can prevent the litigation that often follows the death of a parent of adult children who has remarried.