Linked from
The 10 pages that link to Pig iron, each with the reason it gives.
SteelmakingRelated: It is the principal hot-metal feed for the basic oxygen route.
Cast ironRelated: It is a common molten feedstock for making cast iron.
Bessemer processRelated: It is the molten feedstock whose carbon and impurities the process removes.
Henry BessemerRelated: It was the principal feedstock the Bessemer process converted into steel.