Reptile substrates (also called bedding material) are used to cover the base of the enclosure; there are a multitude of different substrates with different properties to suit different species of animal. Using loose substrate can have many benefits for different species so we would always recommend using an appropriate substrate rather than paper towl etc. Sandy soils promote natural digging and burrowing behaviour in species that inhabit arid areas while substrates like coir, earth mix, and bio life forest are essential for maintaining humidity in tropical set ups.