Write caching could allow the harddisk drive to stay in a lower power non-spinning state for longer periods of time, extending battery life for the notebook computer.
Perhaps the biggest gain may be in dual storage computers where a flash memory device is used for storing operating systems, software and frequently accessed content (and maybe even for write caching) while a harddisk drive provides much less expensive mass storage of user data.
Of course the total energy used by a harddisk drive depends upon the actual usage and power saving modes, and caching of data can reduce the energy used by a HDD in a computer system.