They are an important part of making it bioactive. Which should be the only type of substrate anyone should use. Using something like paper towels, reptibark, or anything else unnatural would quickly become overrun with possibly harmful bacteria(hence why people with barebottom enclosures have to clean constantly). With a proper substrate, beneficial bacteria outcompetes the dangerous and provides a balanced ecosystem of flora and fauna. If you have any experience with saltwater aquariums it is a very similar concept.
Springtails eat mainly mold and fungus, while isopods tend to eat dead feeders, poop, dead leaves, rotting wood, etc. I also add superworms, roaches, millipedes, etc to my substrate, but it's not necessary. Springtails are about the size or smaller than a grain of sugar, you can barely see them until there are a ton of them. They stay in the substrate and generally stay in moist areas. There are different species with different preferences though. Isopods also pretty much stick to the substrate. I've never seen them try to escape though they are larger. I have the giant canyon isopods which can also be used as feeders and they get about the size of a small dubia.