This squat variation places your quads in better alignment with the direction of the resistance (compared to a barbell squat), so you can further isolate them.
It's also a great option if you need to unload your back and take a break from the barbell. Makes for a great finisher too, after standard barbell squats.
Heels-Elevated Cable Squat
Why heels elevated? Well, in Arnold's era, raising your heels and doing squats was a useful way to build your quads and get more depth. But in the '90s, this heel elevation was considered cheating. Today, it's all the rage again because, well, they were right in the '70s.