What It Says On The Box:
Vermiculite is good for where you need stable high humidity for long periods of time. With some South American and Asian species we mix in 20-30% vermiculite with coco fibre to help keep the humidity high. Though bear in mind no spiders like stagnant conditions so make sure you also have a good airflow. We use the same mix for spiderlings as it make burrowing easier.
It is also a popular substrate for Phasmid and Mantis keepers.