What's the back "Power" butttom for?

Hello everyone, greeting from Spain.

I Just bought a WD Essential 2T, USB 3.0, and I have a doubt, I saw there’s a back buttom with the sign of Power (like the on and off devices) what’s it for? I dont see anything about in the intructions… can someone tell me how this buttoms works? or what does it do? or is just decoration ( which I don’t think so)

I think it only works if the Smartware is installed.