The break up of a relationship is stressful and traumatic. It jolts people out of their life styles or brings home to them that their life choices have caused the divorce. Many of Grisham's characters are coping or not coping with these problems while the narrative unfolds. It adds depth to their characters and adds depth to the story.
And being a lawyer can be very stressful. Add the stress of being a litigation or criminal lawyer to your domestic life collapsing and the story has even more depth.