If you need to restore the iPhone to its original state the process requires iTunes. This can be useful for troubleshooting to see if a problem is hardware or software related. The process can also reset an iPhone for resale, wiping all personal data from the device.
Should you suspect a hardware issue with your iOS device, Apple will normally recommend a complete restore to see if this clears up the problem. You can save time when you go to the Apple store by trying a complete restore first. Restoring will update your iPhone firmware to the latest version.