A “fault divorce” is not very common, and most states don’t even recognize them. In the states that do recognize this type of divorce, it is defined as a spouses requesting a divorce be granted on the grounds of some fault on their soon-to-be ex-spouse.
The most common grounds for being granted a fault divorce are adultery; one spouse abandoning the other for a particular length of time; a spouse is physically unable to have sex; one spouse serves a prison sentence; or when a spouse inflicts “emotional or physical pain”, which is referred to as cruelty.
The reason that these are the grounds for a fault divorce is that in each scenario, it should be very easy to show who is at fault. With the ability to more acutely display causation of one of the grounds (for instance, stating which spouse was in prison or was cruel), the courts can address the divorce in a particular manner necessary for that specific divorce.
With a fault divorce, the dynamic of the litigation process shifts based on what must be discovered, and how each spouse will be represented. It also can shorten the duration of a divorce, because with each of the grounds comes an understanding of why the spouses are separating, and what needs to happen from here on. If you want to get a divorce under one of these grounds and do fit one of the listed grounds, then it can be a very good step to take.
Currently, there is no state that requires the spouses seeking a fault divorce to live separately for a specific period, unlike in a no-fault divorce. Proving fault also affords the spouse without fault a larger piece of the marital estate and/or spousal support. These two features make a fault divorce much more appealing to some, but again, with the number of states that do recognize them being so small, fault divorces are rare.