Ferrous sulfate is mainly used as a precursor to other iron compounds. It is a reducing agent, and as such is useful for the reduction of chromate in cement to less toxic Cr(III) compounds.
Historically ferrous sulfate was used in the textile industry for centuries as a dye fixative. It is used historically to blacken leather and as a constituent of ink. The preparation of sulfuric acid ('oil of vitriol') by the distillation of green vitriol (Iron(II) sulfate) has been known for at least 700 years.