Hi
This exercise targets strengthening of the calf muscles. As you move to the right and lift your heels you emphasise concentric strengthening of the right calf muscle and as you shift to the left and lower your heel you emphasise eccentric (lengthening) strengthening of the left calf muscle.
If you want to get a symmetrical effect on both calf muscles you would need to reverse the direction and work the same amount in both directions.
Eccentric strengthening is often prescribed if you have a problem with your Achilles tendon (tendonosis) which attaches the calf muscle to the heel bone (calcaneus)