no puppose.
saves about 5W but cost some MH/s(about 10-40MH/s drop, depend deep of downclocking and GPU model. ~20MH/s for 250 Mhz on 5870, for example).
slightly improve cooling situation for GPU's with badly designed power subsystems[VRM's/IC's without heatsinks and without strong airflow], unsuitable for mining anyway.
if you care about GPU health put extra-fans in case instead. of [better]invest to water-clooling system. for both GPU, GPU subsystem, CPU, mobo chipset, with external radiator[without fan. huge, expensive, but silean and efficent beast]and even with optional heatsink for PSU[relatively rare product]