What is the purpose/use/function of layer cards?

You use them to create other items with. It only works if you have a zombie skin active (from previous plots), though. But you take them to the Alchove in the Shadowglen Underground and you can combine them with tokens (to define what you're making, like clothing or food or beanbags, etc.) and tinctures (to define which colour you want) from the Philosopher's Store to make different items. The whole list of items you can find on Subetalodge here.

Oh, awesome. Thank you for all your help this morning! :)