I'm not sure what's going on with optical mice nowadays, but its strange. Not long after Canon announced its mouse-calculator hybrid, this USB-digital scale has also made its appearance. Capable of measuring weights from 0.1g to 500g, the scale's range of use seem fairly limited - assuming, that is, you don't work in the drug or jewelry industries. But, assuming you do work in the drug or jewelry industries, would you trust your trade to the numbers given to you by a $22 mouse scale? I'd certainly hope not.
But the above mouse isn't alone. I did a bit of research and found a number of other products that are pretty much the same thing. There's the MSC-650 Optical USB Mouse Scale from AmericanWeigh and this "Optical mouse with hidden digital scale and stash compartment." Notably, Amazon notes that buyers of the former also purchased products like clear reclosable zipper bags and wall clock/hidden safe combinations. Suddenly, the target demographic for these things seems a lot more clear.