Why it's on the list: Ab exercises with added resistance don't get enough love! Extra resistance spurs growth in the fast-twitch muscle fibers like almost nothing else and can really build up the "bricks" of your six-pack.
By using a machine, you can also adjust the load and train to failure at just about any rep target you want. A pin-loaded machine works especially well when doing dropsets. If your gym doesn't have a dedicated ab machine, you can still get the same benefits using cables or bands.
Machine Crunch Variations for Ab Development:
In your workout: This works great as one of the first exercises in your ab routine. Use a challenging weight for a lower rep target. Of course, it also works with lighter weight as a burnout at the end. In either case, 3-4 sets of 10-15 reps is a solid formula.