@iclazion
Here are some benefits from some sites:
1."Vermiculite: Vermiculite is aluminum-iron magnesium that is heated and puffed up to form lightweight, sponge-like granules capable of retaining water and air. It is chemically inert, fire resistant, non-toxic, and sterile and does not deteriorate. Vermiculite can be found in various brands of potting soil, lightweight plaster, concrete and insulation.
As for vermiculite as a standalone substrate, doesn't encourage fungus or mites and when damp can hold the form of tarantula burrows. The white/yellow metallic sheen it produces adds a very unique look to an enclosure. When wet it tends to clump and stick to everything including tarantulas. Possibility that this impedes upon some natural behaviors associated with burrowing and climbing"
www.tarantulatour.com/substrate.html
It seems like the only problem I can get from this is that it gets stuck to their feet, but most keepers mix it 50/50 with either coconut or peat whichever they prefer.
Hope this gives a better view on the subject
Substrate
"The purpose of the substrate is to hold moisture to help maintain humidity levels and to allow certain species to construct their burrow. The preferred substrate is vermiculite, either alone or mixed 50/50 with peat or potting soil. Vermiculite alone should have water added until it has the same dampness as, say, your average garden topsoil. While some also recommend wood chips or shredded bark, there are differing schools of thought on this, especially the wood chips, and I prefer to just stick with vermiculite. Sand and gravel make a poor substrate and are generally avoided although some people like to mix it with the peat and vermiculite. Feel free to experiment!
Fill your enclosure about 1/3rd of the way up with substrate material. This will give plenty of moisture and will also prevent injury to the spider by limiting how far it can fall if it decides to get rambunctious."
www.rusticgirls.com/animals/tarantula-care.php
There are many other sites like these and