OptiPlex 7780 All-In-One, small button by power button?

I just received and set up my new All In One. Just one question: The scant paperwork and downloaded manuals say that the power button is on the lower right of the front bezel. I see two buttons, one larger one with the power indicator light that I've been using, and one microscopic button next to it that is too small to touch. What does that little one do and why is it a second one right next to the main button?