Phenyl-hydrazine is used to prepare indoles by the Fischer indole synthesis, which are intermediates in the synthesis of various dyes and pharmaceuticals. 

Phenylhydrazine is toxic by single exposure via the oral route (LD50 80–188 mg/kg body weight) and is expected to be toxic by the inhalation and dermal routes

Phenylhydrazine is used to form phenylhydrazones of natural mixtures of simple sugars in order to render the differing sugars easily separable from each other.

PHENYL HYDRAZINE AR