Erasing everything off of your phone and starting anew is a pretty drastic step to take for a little glitch. For most people, system restoring is a last resort option, but there are a few reasons to want or need to restore your device to factory settings.
Many people have tried to unsuccessfully root their Samsung Galaxy S3, and in turn have bricked their phones. System restore is one of the only options to recover a bricked phone.
If you are simply having hardware issues that you cannot seem to fix on your own, restoring the system is an easy way to solve those problems, too.
Also, if you forgot your password and are locked out of your phone, then you can restore your phone to get access again, though there are ways around it. If you really need to restore your Samsung Galaxy S III (or most other Android smartphones), this is how you do it:
To begin the process, you will remove and then reinsert the battery. After that, press and hold Volume UP, Home, and the Power Button. Hold these three buttons until the phone vibrates and then release the Power Button whilst holding the other two. Hold these two until you see the Android robot. Once you have reached this stage you can choose the Factory Reset option. It's as simple as that.