Ethyltripropylammonium iodide is primarily used in organic synthesis as a phase-transfer catalyst. It facilitates the transfer of reactants between immiscible phases, enhancing reaction rates and yields in processes such as alkylation, esterification, and nucleophilic substitution. Its application is particularly valuable in the production of pharmaceuticals, agrochemicals, and specialty chemicals, where it enables efficient and selective transformations. Additionally, it is employed in the synthesis of ionic liquids, which are utilized in various industrial applications, including electrolytes in batteries and solvents in green chemistry.