Isopods... am I missing something??

I keep seeing how everyone with natural vivarium set-ups has isopods and giant millipedes in it, and everyone always mentions how they help to break down the gecko waste. But I can't find anything that supports them actually EATING the waste, and as far as I'm aware they are all pretty much vegetarians anyways.

Do they just have something to do with the nitrogen cycle of a natural set-up's soil (such as add beneficial bacteria that ITSELF helps break down gecko waste)? Or do they actually eat the waste of fruigivores (such as cresteds) since it does contain some fruit content?

The reason I ask is that I was hoping to put some isopods in my cat gecko set up (which are protein eaters only), but the only natural stuff in there is eco-earth,sphagnum moss, and corkbark. Do I have to have actual rooted plants for them to survive or are they okay with a few inches of eco-earth and large quantities of corkbark and moss? What about if I add natural leaf litter?

Can anyone shine a light for me??