These are obviously tricky since they can create disharmony just when you need trust the most. That said, they can be excellent agreements particularly when there is a second marriage. Either spouse may wish to absolutely retain certain assets should there be a divorce or death. That's because few people know that each state has dower or curtsey laws that allow a surviving spouse access to a certain percentage of the estate unless legally indicated otherwise. That amount is usually 1/3 to ½ of the estate. Under a prenuptial agreement, the parties could agree to relinquish their respective rights under the statutes and accept the amounts per the terms of the agreement. There are usually four standards to such agreements
Prenuptial and even post nuptials actually should be demanded by some family businesses where the parents, as owners, are concerned that non family members might get partial ownership of the business upon the death or- more importantly- upon divorce of their child. Unfortunately, I have seen such lack of agreements and a subsequent divorce make a mockery of the disposition of the business.
Also note that some agreements may be structured to terminate after a period of time has elapsed. It enhances the equity of the partners in a long term marriage.
Note that while these agreements are focused for the traditional marriage, they should be contemplated by same sex relationships since the emotional and financial devastation can almost be as significant.