Scratch Me At (D,7)
On the one hand, Chindogu is a series of Japanese inventions "with the sole purpose of having no purpose." Yet, Kenji Kawakami, originator of chindogu and President of the 10,000-member International Chindogu Society, describes it as “an intellectual game to stimulate anarchic minds.”
Its use certainly justifies knowing the basics of coordinate graphing.
By definition, to be chindogu (or "weird tool") an object cannot be patented, must be anarchic in spirit, must exist, and must not be for sale. As an example, consider the Back Scratcher's T-shirt, which requires the use of mathematics:
I have included more examples of the "art" of Chindogu on the Humor page for this week. If you want to see even more, consult the book The Big Bento Box of Unuseless Japanese Inventions.