The double-freeze kills moth eggs or larvae that might have caused that flaw you can see. You can do it with any material but you should definitely do it with all materials that are particularly vulnerable to moths, which means all animal fibres, so wool (and similar yarns, cashmere, mohair, angora etc), silk, fur and feathers, and blends that include them. I tend to freeze all such items whether they show damage or not, as a precaution. It's not so important with plant fibres (cotton, linen) and synthetics, though it's not impossible for those to get moth damage.