If you install a lot of apps and some of them are acting a little mischievous, or maybe something has just gone wrong deep down in the system where you have little chance of fixing it. Wiping the phone clean and starting over is the best overall option sometimes. What's more, if you are about to sell your phone, you must want all your personal data gone before handling the phone to its new owner. And factory reset could just clean up your data in these cases.
