What are useful purposes for ETC's?

Exchange Traded Commodities (ETCs) are intended to make it easier for investors to get exposure to commodities without trading the underlying futures contracts or taking physical delivery. They can be bought and sold on stock exchanges and bid ask spreads are a lot lower than if buying and selling physical commodities. Also, there is no storage cost, as with physical commodities.

That is the use of ETCs: getting exposure to commodities without the hassle and expense of physical ownership, or even of trading them on a commodities exchange.

If you REALLY want to be able to take physical delivery of your ETC gold fund, you can do that in at least one circumstance.

Deutsche Börse Commodities appears to allow physical delivery of gold ETCs:

When choosing a provider such as Xetra-Gold, where investors have the right to delivery of the securitised amount of physical gold, they can combine the benefits of exchange trading with physical ownership of the commodity.