Not to come off as condescending but we don't know about it because there is so little documentation about it.

Historians can learn a lot by looking at it and its context, but we can't know its purpose, designs, and intents, without some kind of documentation.

In this case, the documentation may exist, but has yet to be found. Alternatively, this project could have just been an attempt by small department within a factory, a prototype, or just deemed an utter failure. In the chaos of war, it is no surprise that documentation about these things get lost. Especially if the Kugelpanzer wasn't ordered by the German Government. (Government records tend to be better kept than private ones.)

I have another question, in the link it says it was shipped to Japan from Germany. How were the Germans able to get anything all the way to Japan during the war? Afterall, the Soviets controlled all the land in between and the British/ Americans controlled all the seas inbetween.

Short answer because its in a Russian museum and the Russians aren't massively forthcoming with details.

One possibility is that the thing is a fake made in the museum or related workshops. Either as a joke or an attempt to track how much information was getting from Kubinka tank museum to the west.

Second is that its real but not a weapon. Some kind of seed spreader perhaps.

Third it is a real and a weapon but not german perhaps either chinese or japanese in origin. Alternatively it could be a soviet student project or the like (I do wounder if it is meant to be deployed on its side like the british smith gun https://en.wikipedia.org/wiki/Smith_Gun )

The claims about it seem to be set up to make it seem if anything more mysterious which makes me strongly susspect they aren't true. Thus I lean towards hoax or student project.

Perhaps someone had had a subscription to Popular Science Monthly.

July 1936, p 37, has the cover story about a "tumbleweed tank" proposed by a Texan. The inventor claimed "the tank's spherical shape presents the smallest possible target for enemy bombs or shells, and all but direct hits would glance off its curved sides." They are shown rolling through barbed wire barriers, and I have to note that they would use less steel than a conventional tank. As well, they would transport more easily and unload with fewer problems at islands with primitive ports.