What is this serrated wrench tool for?
It's a wrench that will work on a range of different size nuts and bolts. Each step on the top ramp will grab a different size. A further advantage is that it will work equally well (or badly) on four sided nuts, and on five and seven sided "security" bolts that a conventional adjustable wrench with parallel jaws will not work on.

It will not work especially well, but it's a one-piece equivalent to a proper adjustable wrench, which in turn will not work as well as a proper set of spanners. They're made because they're cheap, light, and easy to manufacture.

Edit: found the ad on ebay and it has another image:



Since it's only about 10cm long, it seems to be about 2mm thick based on the image above. So Daniel might be right, it would work for holding cones and locknuts in place. Wandering the internet I haven't been able to find anything similar, unfortunately, but I recall seeing them in some collections of very old tools.