What is the sense of LED energy saver?

If you roll your mouse cursor over the “i” graphic it will give a brief explanation of what the option does.

Enabling or disabling the LED option: “Sets the status of your drive’s LED.”

In other words it turns on or off the front LED on the single bay/single drive My Cloud.

Edit to add: It should be noted some have reported the blue element of the front LED burns out. Some have resorted to modifying the My Cloud firmware to use an alternate color. Example of past discussions: