Richway BioMats are designed to be used with a fitted cotton cover at all times. Without the cover, the BioMat actually does not function properly. The main purpose of the cover is to insulate the crystals. This lowers conductive heat transfer, and maximizes infrared heat transfer.
The cotton covers also serve an important secondary function - they protect the BioMat from sweat, oils, and dirt. They are machine washable and dry-able, unlike the BioMat itself.
However, most people do not need to order an extra cotton cover, because the BioMat already includes a fitted cotton cover. But it can be useful to have an extra, so that one cover can be in the laundry while the other is in use.