From Rise: The Vieneo Province
Processed Storage is for secure storage of commodities.
Seed buildings like the Mining Command building and Farmstead have some secure storage, but usually not enough. In addition, commodities that are not stored in secure Processed Storage or in a seed building will have to be stored outside.
Outside the rain will wash it away, it will spoil, and it can get stolen. If you run out of Processed Storage, production will cease.