Post-Marital Agreements are useful especially in marriages where the parties do not legally share any or all children. This can come up in a number of different situations, but the most common are:
When spouses are starting estate planning and want to make a clear declaration that each spouses' separate children shall inherit from their legal parent or guardian
When inheritance is imminent or has recently been received by one spouse from a parent or other relative and spouses want to make sure that it flows to the children of the spouse that inherited the property
Post-Marital Agreements are generally much less contentious than most people imagine, as my experience would show that the couple usually goes into the agreement with a shared goal to protect descendants, such as children and grandchildren.
These agreements can also discuss how assets the parties had upon entering into the marriage are to be handled. This is also helpful in the situation of a marriage without all or any shared children.
Keep in mind, both parties need separate counsel so that each spouse can have someone with their best interests review the document.