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
How to do
Attach a straight bar to the low end of the cable machine, almost at floor level.
Grab the straight bar with an underhand grip using both hands (palms facing upward).
while standing erect with a straight back, slowly curl the bar up to the top position which will be a few inches from your chest.
Slowly return the bar back down to the starting position and repeat.
Don't bend your body to help perform the movement and keep your elbows close to your body in the same position. Do not let the weight plates touch during the exercise, keep the tension on the bicep muscles.