4-Cyanophenylhydrazine hydrochloride is primarily used in organic synthesis as a building block for the preparation of various heterocyclic compounds. It serves as a key intermediate in the synthesis of pharmaceuticals, particularly in the development of drugs targeting neurological disorders and cancer. The compound is also utilized in the production of dyes and pigments, where its hydrazine moiety facilitates the formation of complex colorants. Additionally, it finds application in agrochemical research for the creation of novel pesticides and herbicides. Its reactivity with carbonyl compounds makes it valuable in the synthesis of hydrazones, which are important in analytical chemistry for the detection and quantification of aldehydes and ketones.