There are some hidden settings on your device that may not be changed by a factory reset. All it does is basically reinstall your base OS and wipes out all the apps and data on your device (not the sd card). It's good for clearing out the gunk and to speed things up from time to time. No negative impact on longevity of your device as far as I know.
It's akin to reinstalling Windows on your home PC every few years to clear out the registry and speed up your system. That doesn't hurt your PC, in fact it boosts performance.