The fan may be driven by the engine, but unless the engine is working hard, it is not always needed while the car is moving, so the energy used in driving it wastes fuel.
Umuntu othanda ukuqhutshwa yinjini angaqhutshwa yinjini, kepha ngaphandle kokuthi injini isebenza kanzima, ayidingeka njalo ngenkathi imoto ihamba, ngakho-ke amandla asetshenziswa ekushayeleni imosha uphethiloli.