Linked from
The 6 pages that link to Flux (metallurgy), each with the reason it gives.
SolderingRelated: It clears surface films that would prevent solder from wetting the metal.
FluoriteRelated: Fluorite lowers slag viscosity in metallurgical furnaces.
SmeltingRelated: Limestone and other fluxes bind ore impurities into slag.
Copper smeltingRelated: Silica flux binds iron oxides generated from copper sulfide ores.
SolderRelated: Flux clears surface oxides that would stop molten solder from wetting a joint.