Think of a post-marital agreement as a pre-nup for a married couple.
Just like a pre-nup, a post-marital agreement outlines how assets and debts will be divided in the event of a divorce and it typically addresses spousal support, too. Many couples also use the post-marital agreement process as a way to clarify expectations and responsibilities within the marriage.
By mediating your post-marital agreement, you are trying to save your marriage while protecting yourself, too. It is a way to re-set your marriage by having a third party facilitate a set of difficult discussions while simultaneously drafting-up a legal contract to enforce agreements from that difficult discussion.
If you want to save your marriage, but have (earned) trust issues with your spouse, you may want to consider a post-marital agreement.