I read that a vivarium is an enclosure for the study and observation of creatures in a "semi-natural" environment; plants are not necessarily required. Since these Blatta Orientalis are "city bugs" they don't usually see a lot of green space. But I'm not about to replicate sewers and drains...
Not sure yet about any practical purpose. I've noticed they spend a lot of time walking around the perimeter of the container bottom and I was wondering what they were doing. They have food, water, and places to snuggle in. Are they looking for something or just stretching their legs? They don't mind the wheel, but it doesn't rotate very quickly; they don't weigh enough to get it really spinning. If there are a bunch of them hanging on the bottom of the wheel it doesn't turn at all.
I'll know more tomorrow when I review the time lapse I'm shooting.