The jute Urban Worm Blankets are laid on top of the vermicompost to help regulate moisture, keep conditions dark, and trap a little bit of heat. 
If the vermicompost is wet enough (and it usually) is, this will help trap some of the moisture and you will find that worms are actually attracted to the natural fibers.

The air spaces in in the fibers also act to trap a small amount of heat inside the vermicompost.

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