I've heard of it being done by girls who basically want free, new underwear every day. Some nutter pays them £x and sends them the clothes, they wear it once, seal it in a jiffy bag and post it off. They don't ever have to wash knickers/tights/stockings again.
When I worked at an online hosiery place, there were a few regular orders made by one person for despatch to someone completely different. Some places now do secret gifts on forums attached to online shops.