o-Phenylenediamine dihydrochloride is widely used as a reagent in organic synthesis, particularly in the production of dyes and pigments. It serves as a key intermediate in the manufacture of hair dyes, where it helps in achieving various shades of color. Additionally, it is employed in the synthesis of certain polymers and resins, contributing to their structural properties. In analytical chemistry, it is utilized as a precursor for developing colorimetric assays, aiding in the detection of specific substances. Its role in the preparation of heterocyclic compounds is also significant, as these compounds are often used in pharmaceuticals and agrochemicals. Furthermore, it finds application in the field of biochemistry for studying enzyme activity and protein interactions.