Substrates in natural terrariums have many functions and are not only there for decoration purposes. They play a very important role in the determination of the terrarium's micro-climate.
Digging and burrowing reptiles and amphibians have different needs than tree dwelling species, whereas desert animals prefer yet another type of substrate.
Heat conductivity is important when a substrate heater like the Heat Wave is used and the absorption capacity will contribute to the overall air humidity.
Online Reptile Shop supply a series of substrates for the dry, humid and aquatic terrarium, all made from natural materials.