While free weight exercises are great, there area few advantages using a cable.
They allow you to attain a full range of motion from top to bottom with constant tension directly placed on the muscles.
This exercise also helps to reduce the strain on joints, tendons and ligaments which some people incur from using free weights.
Also it is much more safe and stable motion for people who want to avoid using free weights like barbells and dumbbells. and might reduce the risk of injury.
The Cable curls is an isolation exercise, which works mostly on your biceps, but also uses some stabilizer muscles in your shoulders and lower back