Some ETCs (e.g. ETFs oil securities and ETFs commodity securities) are priced off futures as it is not possible to store the underlying commodity. In addition, futures pricing can be more liquid and efficient for some commodities, especially where the futures contract helps to standardise the pricing, e.g. agricultural commodities where quality can vary between crops, seasons and regions.
In the case of physically backed ETCs, precious metals are homogenous, can be stored easily and do not decay. They can therefore be priced directly off the underlying physical commodity.