If you press it and use the arrow keys you can navigate the entire start menu
that includes shut down/log off.
It's for if your mouse stops working OR if you're in a fullscreen thingy and don't want to close the program.
You can use just the keyboard for a whole bunch of things.
Tab can be used to navigate webpages, and if you rightclick, some things will have keyboard shortcuts listed next to them.
For example, F5 is reload, and CTRL+T is for opening a new tab. On Chrome, anyway.