Why do people use fault divorce when they can end a marriage in easier ways?
Sometimes there is a strategic advantage in fault. In the old days, when Rufus and Rhonda came to the end of the road, Rhonda could hold Rufus’s feet in the fire and make it so difficult for him to divorce her that she effectively trapped him in a marriage they both knew was over. This is Pyrrhic victory for sure, but people did it.
Sometimes people go for fault divorce because it raises the bar. In general, one divorce strategy is the determination in which party’s interest is it to remain married longer. The party who can remain married longest has what lawyer’s call at leverage factor over the other party. If Rufus wants to leave Rhonda for Roxanne, Rhonda make this a weapon against him with a fault divorce charging adultery. And she could drag it easily by making bankrupting demands.
Today, no-fault has changed this. Even in a fault state when one spouse has filed or counterfiled for divorce, "provided that the allegations setting forth the cause for action are true, that spouse will succeed" in the divorce. If, for example, Rufus has committed adultery, he cannot counterfile for divorce claiming her adultery, but he can counterfile on grounds of irreconcilable differences and get a divorce regardless of the fact that Rhonda is blameless.
In short, even in so-called fault jurisdictions, one party cannot prevent the other from divorcing him or her as long as "requisite statutory grounds for divorce are alleged and are provable by a preponderance of evidence," which means a fraction of percent more likely than not.