I would use this when emotions are running high, when the emotions are too intense to be able to do the work. Nothing wrong with emotions and we need to be able to help couples regulate, process and deal with emotions but if it is too intense then it is counter-productive. Using this dialogue can help them remember that they are here for the same reason, help them get back on the same page and as Effie described help them to re-connect.
I would also use the Appreciation Dialogue as a means to practice the formula prior to dealing with frustrations via the Dialogue, so that they can get comfortable with how to make an appointment, how to send, how to mirror, summarize etc. before they have to deal with the tougher stuff.