According to this review, it is for attaching the "Viewing Window" - whatever that is.
http://www.notebookcheck.net/Review-Logitech-Performance-Mouse-MX.49189.0.html
"... The viewing window is secured with a magnet and allows for an initial visual impression of its design. ..."
I bet the package can open a front "flap" - like the cover of a hardcover book - allowing you to see the mouse under a plastic shroud (so you can admire it before actually unsealing the box).
That magnet just holds the flap closed![]()
A small circular magnet about the size and thickness of a standard tylenol/aspirin
What the hell is it for?
Inside the packaging:
1. Mouse
2. MicroUSB charging cable
3. "Manual"
4. Magnet
Not included:
Carrying case like the Anywhere 1![]()