Why focus on smelters/refiners as the key point in the supply chain?

The producers and traders of minerals provide them to SORs and the SORs provide the resulting metals (or intermediates) to a large number of downstream consumers. Smelting/refining is the conversion point of identifiable types of materials, such as ore concentrates and secondary materials, into a common product for all downstream consumers. Because of this transformation process, SORs are in the position to know the origin of the material before it is commingled and processed for distribution to a wide variety of downstream users. Moreover, SORs are small in number relative to the number of upstream suppliers and downstream users. For these reasons, smelters/refiners are identified as the "pinch point" of the supply chain.

Visually, the supply chain looks like an hour glass timer with SORs at the middle:

SOR Hourglass