Not sure if others had the same experience, but many times I've seen that for whatever reason the Main cabin wasn't available but X fares were. If I feel that First class is too expensive and Main isn't available, X fares could be useful. It surely isn't as convenient to fly X but on some short-distance flights it isn't a big deal.

Seat selection flying X would also be useful.

P.S. Thanks for summarizing the suggestions.