I have made a lot of research on this thing (meaning mostly searching on the internet and contacting a few people that also try to find information about it) and tried to figure out by myself what this thing could be for. I have made this post not too long ago hoping that it might help to solve the mystery. Now the conclusion of my research is that we dont know ANYTHING about it besides the famous list of established facts that you see everywhere. Everything else is pure speculation.

Now speculation is still interesting though:

Some people think that this might be a mobile blast shield for a controlled demolition. This would mean that the gun port at the front is actually for the cable of the explosives.

If you take a look at the thing you will quickly realize that it can only move on a smooth surface because it has this tiny wheel at the back that would jump over every pebble and get stuck everywhere. It would be like trying to use a skateboard on the beach. You can also see that little hinge above the wheel which might be used to connect the vehicle to a Kettenrad or something that would tow it. The whole arm on the back might also be a temporary construction because this was probably still in development when it got captured by the soviets.

Most people think that this is a reconnaissance vehicle but I dont think thats likely because it only has 1 vision slit which is tiny. You could see barely anything. It would probably be powered by a small combustion engine which is pretty loud for a 1 man reconnaissance vehicle and the thing would probably be really slow. And a radio probably wouldnt fit in it either.

You would probably be able to do better reconnaissance alone, on foot, with a pistol and some signal flags.

TL:DR No idea what that thing is.