Parchment paper or fabric is put between the jar and jar lid usually to prevent corrosion of the metal lid. This will happen with herbal vinegar’s, and less often with alcoholic tinctures or infusions. Another reason may be that the person making the preparation wants to prevent contamination into the tincture or infusion from chemicals or components of the lid (depending on what it is made of).
I hope this helps clarify that a little bit. Happy herbal learning and welcome to the program!