Why it's on the list: A knock against machine-based leg exercises is that the hormone boost they generate just doesn't compare to free weights. But that's not a reason not to do them! It just means do them later in your leg workout when you're already fatigued.
One of the primary benefits of the hack squat is that you can manipulate foot placement, which is why fitness model Julian "The Quad Guy" Smith considers them one of his signature moves for legs. A high placement allows you to descend farther, which emphasizes the glutes and hamstrings. A lower placement means the quads take up a greater percentage of the workload.
It's also easier to use advanced intensity techniques like forced reps and dropsets, because you can change weight quickly and aren't balancing a barbell.
Hack Squat Variations for Leg Development:
In your workout: Hack squats are typically done after free-weight exercises and before single-joint leg movements. The sweet spot is somewhere around 3 sets of 8-12 reps, although if you're following a program built by a hack devotee like Kris Gethin in his 8-Week Hardcore training plan, you can expect that to go up to 20-30 reps and beyond. Consider yourself warned!
