I've found out that it does do something....
But only when the system is powered, but switched off. Press the button then and only whilst pressed, the system seems to power up. It's item 7 (above).
I had a strange thing happen today....
Something, not me, swapped the left for the right mouse button. I couldn't do a thing with it for an hour, until I eventually worked out that the buttons had been swapped over. I was on the verge of doing a restore, thinking that Windows had been corrupted.