It would be wonderful if every marriage ended happily. But the reality is different.
A Jewish marriage can be dissolved only through a religious writ of divorce, called a get. For a get to be valid according to Jewish law, the husband must give it and the wife must receive it - both of their own free will. Thus, the get can become a weapon in the hands of individuals who want to manipulate a divorce settlement in their favor, or act out feelings of bitterness against their spouse.
The effects of get abuse can be devastating. Most often, women are the hapless victims of recalcitrant husbands who refuse to give a get. That leaves them endlessly locked in dead or abusive marriages or forced to succumb to the extortionary demands of their husbands.
The purpose of a prenuptial agreement, formulated in accordance with halakha, is to prevent blackmail by get refusal. In so doing, the prenup facilitates the dissolution of a marriage so that each partner can get on with their lives.
CWJ recommends its Contract for a Just and Fair Marriage.
Other prenuptial agreements can be found here.